diff --git a/consensus/ssz_types/src/bitfield.rs b/consensus/ssz_types/src/bitfield.rs
index 599170fa2..b0cf4551e 100644
--- a/consensus/ssz_types/src/bitfield.rs
+++ b/consensus/ssz_types/src/bitfield.rs
@@ -620,7 +620,7 @@ impl<N: Unsigned + Clone> tree_hash::TreeHash for Bitfield<Variable<N>> {
         tree_hash::TreeHashType::List
     }
 
-    fn tree_hash_packed_encoding(&self) -> Vec<u8> {
+    fn tree_hash_packed_encoding(&self) -> tree_hash::PackedEncoding {
         unreachable!("List should never be packed.")
     }
 
@@ -641,7 +641,7 @@ impl<N: Unsigned + Clone> tree_hash::TreeHash for Bitfield<Fixed<N>> {
         tree_hash::TreeHashType::Vector
     }
 
-    fn tree_hash_packed_encoding(&self) -> Vec<u8> {
+    fn tree_hash_packed_encoding(&self) -> tree_hash::PackedEncoding {
         unreachable!("Vector should never be packed.")
     }
 
diff --git a/consensus/ssz_types/src/fixed_vector.rs b/consensus/ssz_types/src/fixed_vector.rs
index 5f7a4af96..e64e76ef4 100644
--- a/consensus/ssz_types/src/fixed_vector.rs
+++ b/consensus/ssz_types/src/fixed_vector.rs
@@ -167,7 +167,7 @@ where
         tree_hash::TreeHashType::Vector
     }
 
-    fn tree_hash_packed_encoding(&self) -> Vec<u8> {
+    fn tree_hash_packed_encoding(&self) -> tree_hash::PackedEncoding {
         unreachable!("Vector should never be packed.")
     }
 
diff --git a/consensus/ssz_types/src/variable_list.rs b/consensus/ssz_types/src/variable_list.rs
index 49f8004b2..f23872c87 100644
--- a/consensus/ssz_types/src/variable_list.rs
+++ b/consensus/ssz_types/src/variable_list.rs
@@ -184,7 +184,7 @@ where
         tree_hash::TreeHashType::List
     }
 
-    fn tree_hash_packed_encoding(&self) -> Vec<u8> {
+    fn tree_hash_packed_encoding(&self) -> tree_hash::PackedEncoding {
         unreachable!("List should never be packed.")
     }
 
diff --git a/consensus/tree_hash/src/impls.rs b/consensus/tree_hash/src/impls.rs
index 00fed489c..cf05d2a3d 100644
--- a/consensus/tree_hash/src/impls.rs
+++ b/consensus/tree_hash/src/impls.rs
@@ -14,8 +14,8 @@ macro_rules! impl_for_bitsize {
                 TreeHashType::Basic
             }
 
-            fn tree_hash_packed_encoding(&self) -> Vec<u8> {
-                self.to_le_bytes().to_vec()
+            fn tree_hash_packed_encoding(&self) -> PackedEncoding {
+                PackedEncoding::from_slice(&self.to_le_bytes())
             }
 
             fn tree_hash_packing_factor() -> usize {
@@ -41,7 +41,7 @@ impl TreeHash for bool {
         TreeHashType::Basic
     }
 
-    fn tree_hash_packed_encoding(&self) -> Vec<u8> {
+    fn tree_hash_packed_encoding(&self) -> PackedEncoding {
         (*self as u8).tree_hash_packed_encoding()
     }
 
@@ -62,7 +62,7 @@ macro_rules! impl_for_lt_32byte_u8_array {
                 TreeHashType::Vector
             }
 
-            fn tree_hash_packed_encoding(&self) -> Vec<u8> {
+            fn tree_hash_packed_encoding(&self) -> PackedEncoding {
                 unreachable!("bytesN should never be packed.")
             }
 
@@ -87,10 +87,10 @@ impl TreeHash for U128 {
         TreeHashType::Basic
     }
 
-    fn tree_hash_packed_encoding(&self) -> Vec<u8> {
-        let mut result = vec![0; 16];
+    fn tree_hash_packed_encoding(&self) -> PackedEncoding {
+        let mut result = [0; 16];
         self.to_little_endian(&mut result);
-        result
+        PackedEncoding::from_slice(&result)
     }
 
     fn tree_hash_packing_factor() -> usize {
@@ -109,10 +109,10 @@ impl TreeHash for U256 {
         TreeHashType::Basic
     }
 
-    fn tree_hash_packed_encoding(&self) -> Vec<u8> {
-        let mut result = vec![0; 32];
+    fn tree_hash_packed_encoding(&self) -> PackedEncoding {
+        let mut result = [0; 32];
         self.to_little_endian(&mut result);
-        result
+        PackedEncoding::from_slice(&result)
     }
 
     fn tree_hash_packing_factor() -> usize {
@@ -131,10 +131,10 @@ impl TreeHash for H160 {
         TreeHashType::Vector
     }
 
-    fn tree_hash_packed_encoding(&self) -> Vec<u8> {
-        let mut result = vec![0; 32];
+    fn tree_hash_packed_encoding(&self) -> PackedEncoding {
+        let mut result = [0; 32];
         result[0..20].copy_from_slice(self.as_bytes());
-        result
+        PackedEncoding::from_slice(&result)
     }
 
     fn tree_hash_packing_factor() -> usize {
@@ -153,8 +153,8 @@ impl TreeHash for H256 {
         TreeHashType::Vector
     }
 
-    fn tree_hash_packed_encoding(&self) -> Vec<u8> {
-        self.as_bytes().to_vec()
+    fn tree_hash_packed_encoding(&self) -> PackedEncoding {
+        PackedEncoding::from_slice(self.as_bytes())
     }
 
     fn tree_hash_packing_factor() -> usize {
diff --git a/consensus/tree_hash/src/lib.rs b/consensus/tree_hash/src/lib.rs
index 997ae867f..ec40de916 100644
--- a/consensus/tree_hash/src/lib.rs
+++ b/consensus/tree_hash/src/lib.rs
@@ -8,13 +8,16 @@ pub use merkleize_padded::merkleize_padded;
 pub use merkleize_standard::merkleize_standard;
 
 use eth2_hashing::{hash_fixed, ZERO_HASHES, ZERO_HASHES_MAX_INDEX};
+use smallvec::SmallVec;
 
 pub const BYTES_PER_CHUNK: usize = 32;
 pub const HASHSIZE: usize = 32;
 pub const MERKLE_HASH_CHUNK: usize = 2 * BYTES_PER_CHUNK;
 pub const MAX_UNION_SELECTOR: u8 = 127;
+pub const SMALLVEC_SIZE: usize = 32;
 
 pub type Hash256 = ethereum_types::H256;
+pub type PackedEncoding = SmallVec<[u8; SMALLVEC_SIZE]>;
 
 /// Convenience method for `MerkleHasher` which also provides some fast-paths for small trees.
 ///
@@ -109,7 +112,7 @@ pub enum TreeHashType {
 pub trait TreeHash {
     fn tree_hash_type() -> TreeHashType;
 
-    fn tree_hash_packed_encoding(&self) -> Vec<u8>;
+    fn tree_hash_packed_encoding(&self) -> PackedEncoding;
 
     fn tree_hash_packing_factor() -> usize;
 
@@ -125,7 +128,7 @@ where
         T::tree_hash_type()
     }
 
-    fn tree_hash_packed_encoding(&self) -> Vec<u8> {
+    fn tree_hash_packed_encoding(&self) -> PackedEncoding {
         T::tree_hash_packed_encoding(*self)
     }
 
@@ -146,7 +149,7 @@ macro_rules! tree_hash_ssz_encoding_as_vector {
                 tree_hash::TreeHashType::Vector
             }
 
-            fn tree_hash_packed_encoding(&self) -> Vec<u8> {
+            fn tree_hash_packed_encoding(&self) -> PackedEncoding {
                 unreachable!("Vector should never be packed.")
             }
 
@@ -169,7 +172,7 @@ macro_rules! tree_hash_ssz_encoding_as_list {
                 tree_hash::TreeHashType::List
             }
 
-            fn tree_hash_packed_encoding(&self) -> Vec<u8> {
+            fn tree_hash_packed_encoding(&self) -> PackedEncoding {
                 unreachable!("List should never be packed.")
             }
 
diff --git a/consensus/tree_hash/tests/tests.rs b/consensus/tree_hash/tests/tests.rs
index b7f7178d0..8b2a4b21b 100644
--- a/consensus/tree_hash/tests/tests.rs
+++ b/consensus/tree_hash/tests/tests.rs
@@ -1,5 +1,5 @@
 use ssz_derive::Encode;
-use tree_hash::{Hash256, MerkleHasher, TreeHash, BYTES_PER_CHUNK};
+use tree_hash::{Hash256, MerkleHasher, PackedEncoding, TreeHash, BYTES_PER_CHUNK};
 use tree_hash_derive::TreeHash;
 
 #[derive(Encode)]
@@ -18,7 +18,7 @@ impl tree_hash::TreeHash for HashVec {
         tree_hash::TreeHashType::List
     }
 
-    fn tree_hash_packed_encoding(&self) -> Vec<u8> {
+    fn tree_hash_packed_encoding(&self) -> PackedEncoding {
         unreachable!("List should never be packed.")
     }
 
diff --git a/consensus/tree_hash_derive/src/lib.rs b/consensus/tree_hash_derive/src/lib.rs
index f65be1b6b..21ff324d5 100644
--- a/consensus/tree_hash_derive/src/lib.rs
+++ b/consensus/tree_hash_derive/src/lib.rs
@@ -150,7 +150,7 @@ fn tree_hash_derive_struct(item: &DeriveInput, struct_data: &DataStruct) -> Toke
                 tree_hash::TreeHashType::Container
             }
 
-            fn tree_hash_packed_encoding(&self) -> Vec<u8> {
+            fn tree_hash_packed_encoding(&self) -> tree_hash::PackedEncoding {
                 unreachable!("Struct should never be packed.")
             }
 
@@ -231,7 +231,7 @@ fn tree_hash_derive_enum_transparent(
                 tree_hash::TreeHashType::Container
             }
 
-            fn tree_hash_packed_encoding(&self) -> Vec<u8> {
+            fn tree_hash_packed_encoding(&self) -> tree_hash::PackedEncoding {
                 unreachable!("Enum should never be packed")
             }
 
@@ -288,7 +288,7 @@ fn tree_hash_derive_enum_union(derive_input: &DeriveInput, enum_data: &DataEnum)
                 tree_hash::TreeHashType::Container
             }
 
-            fn tree_hash_packed_encoding(&self) -> Vec<u8> {
+            fn tree_hash_packed_encoding(&self) -> tree_hash::PackedEncoding {
                 unreachable!("Enum should never be packed")
             }
 
diff --git a/consensus/types/src/execution_block_hash.rs b/consensus/types/src/execution_block_hash.rs
index 978bd4c69..988dcece5 100644
--- a/consensus/types/src/execution_block_hash.rs
+++ b/consensus/types/src/execution_block_hash.rs
@@ -67,7 +67,7 @@ impl tree_hash::TreeHash for ExecutionBlockHash {
         Hash256::tree_hash_type()
     }
 
-    fn tree_hash_packed_encoding(&self) -> Vec<u8> {
+    fn tree_hash_packed_encoding(&self) -> tree_hash::PackedEncoding {
         self.0.tree_hash_packed_encoding()
     }
 
diff --git a/consensus/types/src/graffiti.rs b/consensus/types/src/graffiti.rs
index f5f74b601..73beb8264 100644
--- a/consensus/types/src/graffiti.rs
+++ b/consensus/types/src/graffiti.rs
@@ -7,7 +7,7 @@ use serde::{de::Error, Deserialize, Deserializer, Serialize, Serializer};
 use ssz::{Decode, DecodeError, Encode};
 use std::fmt;
 use std::str::FromStr;
-use tree_hash::TreeHash;
+use tree_hash::{PackedEncoding, TreeHash};
 
 pub const GRAFFITI_BYTES_LEN: usize = 32;
 
@@ -159,7 +159,7 @@ impl TreeHash for Graffiti {
         <[u8; GRAFFITI_BYTES_LEN]>::tree_hash_type()
     }
 
-    fn tree_hash_packed_encoding(&self) -> Vec<u8> {
+    fn tree_hash_packed_encoding(&self) -> PackedEncoding {
         self.0.tree_hash_packed_encoding()
     }
 
diff --git a/consensus/types/src/participation_flags.rs b/consensus/types/src/participation_flags.rs
index 74c2cf73b..a2dd49486 100644
--- a/consensus/types/src/participation_flags.rs
+++ b/consensus/types/src/participation_flags.rs
@@ -3,7 +3,7 @@ use safe_arith::{ArithError, SafeArith};
 use serde_derive::{Deserialize, Serialize};
 use ssz::{Decode, DecodeError, Encode};
 use test_random_derive::TestRandom;
-use tree_hash::{TreeHash, TreeHashType};
+use tree_hash::{PackedEncoding, TreeHash, TreeHashType};
 
 #[derive(Debug, Default, Clone, Copy, PartialEq, Deserialize, Serialize, TestRandom)]
 #[serde(transparent)]
@@ -78,7 +78,7 @@ impl TreeHash for ParticipationFlags {
         u8::tree_hash_type()
     }
 
-    fn tree_hash_packed_encoding(&self) -> Vec<u8> {
+    fn tree_hash_packed_encoding(&self) -> PackedEncoding {
         self.bits.tree_hash_packed_encoding()
     }
 
diff --git a/consensus/types/src/payload.rs b/consensus/types/src/payload.rs
index 114ca02ec..667fff58c 100644
--- a/consensus/types/src/payload.rs
+++ b/consensus/types/src/payload.rs
@@ -7,7 +7,7 @@ use std::convert::TryFrom;
 use std::fmt::Debug;
 use std::hash::Hash;
 use test_random_derive::TestRandom;
-use tree_hash::TreeHash;
+use tree_hash::{PackedEncoding, TreeHash};
 
 #[derive(Debug)]
 pub enum BlockType {
@@ -175,7 +175,7 @@ impl<T: EthSpec> TreeHash for BlindedPayload<T> {
         <ExecutionPayloadHeader<T>>::tree_hash_type()
     }
 
-    fn tree_hash_packed_encoding(&self) -> Vec<u8> {
+    fn tree_hash_packed_encoding(&self) -> PackedEncoding {
         self.execution_payload_header.tree_hash_packed_encoding()
     }
 
@@ -244,7 +244,7 @@ impl<T: EthSpec> TreeHash for FullPayload<T> {
         <ExecutionPayload<T>>::tree_hash_type()
     }
 
-    fn tree_hash_packed_encoding(&self) -> Vec<u8> {
+    fn tree_hash_packed_encoding(&self) -> tree_hash::PackedEncoding {
         self.execution_payload.tree_hash_packed_encoding()
     }
 
diff --git a/consensus/types/src/slot_epoch.rs b/consensus/types/src/slot_epoch.rs
index e68055d34..277aa9dea 100644
--- a/consensus/types/src/slot_epoch.rs
+++ b/consensus/types/src/slot_epoch.rs
@@ -16,7 +16,7 @@ use crate::{ChainSpec, SignedRoot};
 use rand::RngCore;
 use safe_arith::{ArithError, SafeArith};
 use serde_derive::{Deserialize, Serialize};
-use ssz::{ssz_encode, Decode, DecodeError, Encode};
+use ssz::{Decode, DecodeError, Encode};
 use std::fmt;
 use std::hash::Hash;
 use std::iter::Iterator;
diff --git a/consensus/types/src/slot_epoch_macros.rs b/consensus/types/src/slot_epoch_macros.rs
index b8e6202fe..fafc455ef 100644
--- a/consensus/types/src/slot_epoch_macros.rs
+++ b/consensus/types/src/slot_epoch_macros.rs
@@ -290,8 +290,8 @@ macro_rules! impl_ssz {
                 tree_hash::TreeHashType::Basic
             }
 
-            fn tree_hash_packed_encoding(&self) -> Vec<u8> {
-                ssz_encode(self)
+            fn tree_hash_packed_encoding(&self) -> tree_hash::PackedEncoding {
+                self.0.tree_hash_packed_encoding()
             }
 
             fn tree_hash_packing_factor() -> usize {
diff --git a/crypto/bls/src/macros.rs b/crypto/bls/src/macros.rs
index a5fce70a9..f3a7374ba 100644
--- a/crypto/bls/src/macros.rs
+++ b/crypto/bls/src/macros.rs
@@ -7,7 +7,7 @@ macro_rules! impl_tree_hash {
             tree_hash::TreeHashType::Vector
         }
 
-        fn tree_hash_packed_encoding(&self) -> Vec<u8> {
+        fn tree_hash_packed_encoding(&self) -> tree_hash::PackedEncoding {
             unreachable!("Vector should never be packed.")
         }
 
diff --git a/testing/ef_tests/src/cases/common.rs b/testing/ef_tests/src/cases/common.rs
index ade8711cd..e77e56193 100644
--- a/testing/ef_tests/src/cases/common.rs
+++ b/testing/ef_tests/src/cases/common.rs
@@ -43,7 +43,7 @@ macro_rules! uint_wrapper {
                 <$wrapped_type>::tree_hash_type()
             }
 
-            fn tree_hash_packed_encoding(&self) -> Vec<u8> {
+            fn tree_hash_packed_encoding(&self) -> tree_hash::PackedEncoding {
                 self.x.tree_hash_packed_encoding()
             }