diff --git a/internal/wire/header.go b/internal/wire/header.go
index 1274e83da..678a04a24 100644
--- a/internal/wire/header.go
+++ b/internal/wire/header.go
@@ -1,7 +1,6 @@
 package wire
 
 import (
-	"bytes"
 	"encoding/binary"
 	"errors"
 	"fmt"
@@ -40,37 +39,27 @@ func ParseConnectionID(data []byte, shortHeaderConnIDLen int) (protocol.Connecti
 // https://datatracker.ietf.org/doc/html/rfc8999#section-5.1.
 // This function should only be called on Long Header packets for which we don't support the version.
 func ParseArbitraryLenConnectionIDs(data []byte) (bytesParsed int, dest, src protocol.ArbitraryLenConnectionID, _ error) {
-	r := bytes.NewReader(data)
-	remaining := r.Len()
-	src, dest, err := parseArbitraryLenConnectionIDs(r)
-	return remaining - r.Len(), src, dest, err
-}
-
-func parseArbitraryLenConnectionIDs(r *bytes.Reader) (dest, src protocol.ArbitraryLenConnectionID, _ error) {
-	r.Seek(5, io.SeekStart) // skip first byte and version field
-	destConnIDLen, err := r.ReadByte()
-	if err != nil {
-		return nil, nil, err
+	startLen := len(data)
+	if len(data) < 6 {
+		return 0, nil, nil, io.EOF
 	}
+	data = data[5:] // skip first byte and version field
+	destConnIDLen := data[0]
+	data = data[1:]
 	destConnID := make(protocol.ArbitraryLenConnectionID, destConnIDLen)
-	if _, err := io.ReadFull(r, destConnID); err != nil {
-		if err == io.ErrUnexpectedEOF {
-			err = io.EOF
-		}
-		return nil, nil, err
+	if len(data) < int(destConnIDLen)+1 {
+		return 0, nil, nil, io.EOF
 	}
-	srcConnIDLen, err := r.ReadByte()
-	if err != nil {
-		return nil, nil, err
+	copy(destConnID, data)
+	data = data[destConnIDLen:]
+	srcConnIDLen := data[0]
+	data = data[1:]
+	if len(data) < int(srcConnIDLen) {
+		return 0, nil, nil, io.EOF
 	}
 	srcConnID := make(protocol.ArbitraryLenConnectionID, srcConnIDLen)
-	if _, err := io.ReadFull(r, srcConnID); err != nil {
-		if err == io.ErrUnexpectedEOF {
-			err = io.EOF
-		}
-		return nil, nil, err
-	}
-	return destConnID, srcConnID, nil
+	copy(srcConnID, data)
+	return startLen - len(data) + int(srcConnIDLen), destConnID, srcConnID, nil
 }
 
 func IsPotentialQUICPacket(firstByte byte) bool {
diff --git a/internal/wire/header_test.go b/internal/wire/header_test.go
index 64fdba560..aad1eef00 100644
--- a/internal/wire/header_test.go
+++ b/internal/wire/header_test.go
@@ -575,3 +575,39 @@ func BenchmarkParseRetry(b *testing.B) {
 		}
 	}
 }
+
+func BenchmarkArbitraryHeaderParsing(b *testing.B) {
+	b.Run("dest 8/ src 10", func(b *testing.B) { benchmarkArbitraryHeaderParsing(b, 8, 10) })
+	b.Run("dest 20 / src 20", func(b *testing.B) { benchmarkArbitraryHeaderParsing(b, 20, 20) })
+	b.Run("dest 100 / src 150", func(b *testing.B) { benchmarkArbitraryHeaderParsing(b, 100, 150) })
+}
+
+func benchmarkArbitraryHeaderParsing(b *testing.B, destLen, srcLen int) {
+	destConnID := make([]byte, destLen)
+	rand.Read(destConnID)
+	srcConnID := make([]byte, srcLen)
+	rand.Read(srcConnID)
+	buf := []byte{0x80, 1, 2, 3, 4}
+	buf = append(buf, uint8(destLen))
+	buf = append(buf, destConnID...)
+	buf = append(buf, uint8(srcLen))
+	buf = append(buf, srcConnID...)
+
+	b.ResetTimer()
+	b.ReportAllocs()
+	for i := 0; i < b.N; i++ {
+		parsed, d, s, err := ParseArbitraryLenConnectionIDs(buf)
+		if err != nil {
+			b.Fatal(err)
+		}
+		if parsed != len(buf) {
+			b.Fatal("expected to parse entire slice")
+		}
+		if !bytes.Equal(destConnID, d.Bytes()) {
+			b.Fatalf("destination connection IDs don't match: %v vs %v", destConnID, d.Bytes())
+		}
+		if !bytes.Equal(srcConnID, s.Bytes()) {
+			b.Fatalf("source connection IDs don't match: %v vs %v", srcConnID, s.Bytes())
+		}
+	}
+}
diff --git a/quicvarint/varint_test.go b/quicvarint/varint_test.go
index 104b0e620..6665f2ecb 100644
--- a/quicvarint/varint_test.go
+++ b/quicvarint/varint_test.go
@@ -313,3 +313,24 @@ func benchmarkParse(b *testing.B, inputs []benchmarkValue) {
 		}
 	}
 }
+
+func BenchmarkAppend(b *testing.B) {
+	b.Run("1-byte", func(b *testing.B) { benchmarkAppend(b, randomValues(min(b.N, 1024), maxVarInt1)) })
+	b.Run("2-byte", func(b *testing.B) { benchmarkAppend(b, randomValues(min(b.N, 1024), maxVarInt2)) })
+	b.Run("4-byte", func(b *testing.B) { benchmarkAppend(b, randomValues(min(b.N, 1024), maxVarInt4)) })
+	b.Run("8-byte", func(b *testing.B) { benchmarkAppend(b, randomValues(min(b.N, 1024), maxVarInt8)) })
+}
+
+func benchmarkAppend(b *testing.B, inputs []benchmarkValue) {
+	buf := make([]byte, 8)
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		buf = buf[:0]
+		index := i % 1024
+		buf = Append(buf, inputs[index].v)
+
+		if !bytes.Equal(buf, inputs[index].b) {
+			b.Fatalf("expected to write %v, wrote %v", inputs[i].b, buf)
+		}
+	}
+}