diff --git a/board.go b/board.go
index a61718d..d99e4c8 100644
--- a/board.go
+++ b/board.go
@@ -4,10 +4,19 @@ import (
 	"bytes"
 	"encoding/binary"
 	"errors"
-	"strconv"
 	"strings"
 )
 
+var fenReplacer = strings.NewReplacer(
+	"11111111", "8",
+	"1111111", "7",
+	"111111", "6",
+	"11111", "5",
+	"1111", "4",
+	"111", "3",
+	"11", "2",
+)
+
 // A Board represents a chess board and its relationship between squares and pieces.
 type Board struct {
 	bbWhiteKing   bitboard
@@ -128,27 +137,23 @@ func (b *Board) Draw() string {
 // String implements the fmt.Stringer interface and returns
 // a string in the FEN board format: rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR
 func (b *Board) String() string {
-	fen := ""
+	var fenBuilder strings.Builder
 	for r := 7; r >= 0; r-- {
 		for f := 0; f < numOfSquaresInRow; f++ {
 			sq := NewSquare(File(f), Rank(r))
 			p := b.Piece(sq)
 			if p != NoPiece {
-				fen += p.getFENChar()
+				fenBuilder.WriteString(p.getFENChar())
 			} else {
-				fen += "1"
+				fenBuilder.WriteString("1")
 			}
 		}
 		if r != 0 {
-			fen += "/"
+			fenBuilder.WriteString("/")
 		}
 	}
-	for i := 8; i > 1; i-- {
-		repeatStr := strings.Repeat("1", i)
-		countStr := strconv.Itoa(i)
-		fen = strings.Replace(fen, repeatStr, countStr, -1)
-	}
-	return fen
+
+	return fenReplacer.Replace(fenBuilder.String())
 }
 
 // Piece returns the piece for the given square.
diff --git a/board_test.go b/board_test.go
index 275988e..74848fe 100644
--- a/board_test.go
+++ b/board_test.go
@@ -5,17 +5,37 @@ import (
 )
 
 func TestBoardTextSerialization(t *testing.T) {
-	fen := "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR"
-	b := &Board{}
-	if err := b.UnmarshalText([]byte(fen)); err != nil {
-		t.Fatal("recieved unexpected error", err)
+	for _, fen := range []string{
+		"rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR",
+		"rnbqkb1r/pp1p1np1/4pp2/2pP3p/8/2N2NP1/PPP1PP1P/R1BQKBR1",
+	} {
+		t.Run(fen, func(t *testing.T) {
+			b := &Board{}
+			if err := b.UnmarshalText([]byte(fen)); err != nil {
+				t.Fatal("recieved unexpected error", err)
+			}
+			txt, err := b.MarshalText()
+			if err != nil {
+				t.Fatal("recieved unexpected error", err)
+			}
+			if fen != string(txt) {
+				t.Fatalf("fen expected board string %s but got %s", fen, string(txt))
+			}
+		})
 	}
-	txt, err := b.MarshalText()
-	if err != nil {
-		t.Fatal("recieved unexpected error", err)
+
+}
+
+func BenchmarkBoardTextSerialization(b *testing.B) {
+	fen := "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR"
+	board := &Board{}
+	if err := board.UnmarshalText([]byte(fen)); err != nil {
+		b.Fatal("recieved unexpected error", err)
 	}
-	if fen != string(txt) {
-		t.Fatalf("fen expected board string %s but got %s", fen, string(txt))
+
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		board.String()
 	}
 }
 
diff --git a/fen.go b/fen.go
index 62ae0cc..115deff 100644
--- a/fen.go
+++ b/fen.go
@@ -137,9 +137,18 @@ var (
 		"n": BlackKnight,
 		"p": BlackPawn,
 	}
+	pieceFenMap = reversePieceMap()
 
 	fenTurnMap = map[string]Color{
 		"w": White,
 		"b": Black,
 	}
 )
+
+func reversePieceMap() map[Piece]string {
+	res := make(map[Piece]string)
+	for k, v := range fenPieceMap {
+		res[v] = k
+	}
+	return res
+}
diff --git a/piece.go b/piece.go
index 1063ba1..e0e20a8 100644
--- a/piece.go
+++ b/piece.go
@@ -179,10 +179,9 @@ var (
 )
 
 func (p Piece) getFENChar() string {
-	for key, piece := range fenPieceMap {
-		if piece == p {
-			return key
-		}
+	res, ok := pieceFenMap[p]
+	if !ok {
+		return ""
 	}
-	return ""
+	return res
 }