diff --git a/chain/blocksync/client.go b/chain/blocksync/client.go
index f5a88a525..38e1f6d2c 100644
--- a/chain/blocksync/client.go
+++ b/chain/blocksync/client.go
@@ -308,6 +308,12 @@ func (client *BlockSync) GetChainMessages(
 	length uint64,
 ) ([]*CompactedMessages, error) {
 	ctx, span := trace.StartSpan(ctx, "GetChainMessages")
+	if span.IsRecordingEvents() {
+		span.AddAttributes(
+			trace.StringAttribute("tipset", fmt.Sprint(head.Cids())),
+			trace.Int64Attribute("count", int64(length)),
+		)
+	}
 	defer span.End()
 
 	req := &Request{
diff --git a/chain/sync.go b/chain/sync.go
index b0ae185b0..eae5dfcff 100644
--- a/chain/sync.go
+++ b/chain/sync.go
@@ -7,6 +7,7 @@ import (
 	"fmt"
 	"os"
 	"sort"
+	"strconv"
 	"strings"
 	"time"
 
@@ -52,6 +53,19 @@ import (
 //the theoretical max height based on systime are quickly rejected
 const MaxHeightDrift = 5
 
+var defaultMessageFetchWindowSize = 200
+
+func init() {
+	if s := os.Getenv("LOTUS_BSYNC_MSG_WINDOW"); s != "" {
+		val, err := strconv.Atoi(s)
+		if err != nil {
+			log.Errorf("failed to parse LOTUS_BSYNC_MSG_WINDOW: %s", err)
+			return
+		}
+		defaultMessageFetchWindowSize = val
+	}
+}
+
 var log = logging.Logger("chain")
 
 var LocalIncoming = "incoming"
@@ -1399,7 +1413,7 @@ func (syncer *Syncer) iterFullTipsets(ctx context.Context, headers []*types.TipS
 
 	span.AddAttributes(trace.Int64Attribute("num_headers", int64(len(headers))))
 
-	windowSize := 200
+	windowSize := defaultMessageFetchWindowSize
 	for i := len(headers) - 1; i >= 0; {
 		fts, err := syncer.store.TryFillTipSet(headers[i])
 		if err != nil {
diff --git a/documentation/en/environment-vars.md b/documentation/en/environment-vars.md
index f5247af1a..9d455a74d 100644
--- a/documentation/en/environment-vars.md
+++ b/documentation/en/environment-vars.md
@@ -48,6 +48,11 @@ block heights that are very far from the current chain height, you may want to
 increase this.
 
 
+### `LOTUS_BSYNC_MSG_WINDOW`
+
+Set the initial maximum window size for message fetching blocksync requests. If
+you have a slower internet connection and are having trouble syncing, you might
+try lowering this down to 10-20 for a 'poor' internet connection.
 
 ## Lotus Miner