diff --git a/.github/workflows/go.yml b/.github/workflows/go.yml
index e481b6f..c8a76d7 100644
--- a/.github/workflows/go.yml
+++ b/.github/workflows/go.yml
@@ -31,6 +31,18 @@ jobs:
         GOGC: 1
     - name: test with race detector
       run: go test -v -race ./ -count=1
+  bench:
+    name: Benchmark
+    runs-on: ubuntu-latest
+    steps:
+      - name: setup Go
+        uses: actions/setup-go@v2
+        with:
+          go-version: 1.16
+      - name: checkout
+        uses: actions/checkout@v2
+      - name: run benchmark
+        run: cd benchmarks && go test -bench .
   coverage:
     name: Coverage
     runs-on: ubuntu-latest
diff --git a/benchmarks/slow_reader_test.go b/benchmarks/slow_reader_test.go
new file mode 100644
index 0000000..bd9623c
--- /dev/null
+++ b/benchmarks/slow_reader_test.go
@@ -0,0 +1,78 @@
+package benchmark
+
+import (
+	"bytes"
+	"encoding/json"
+	"fmt"
+	"reflect"
+	"testing"
+
+	gojson "github.com/goccy/go-json"
+)
+
+// Benchmark decoding from a slow io.Reader that never fills the buffer completely
+func Benchmark_Decode_SlowReader_EncodingJson(b *testing.B) {
+	var expected LargePayload
+	if err := json.Unmarshal(LargeFixture, &expected); err != nil {
+		b.Fatal(err)
+	}
+	for _, chunkSize := range [5]int{16384, 4096, 1024, 256, 64} {
+		b.Run(fmt.Sprintf("chunksize %v", chunkSize), func(b *testing.B) {
+			b.ReportAllocs()
+			for i := 0; i < b.N; i++ {
+				index = 0
+				var got LargePayload
+				if err := json.NewDecoder(slowReader{chunkSize: chunkSize}).Decode(&got); err != nil {
+					b.Fatal(err)
+				}
+				if !reflect.DeepEqual(expected, got) {
+					b.Fatalf("failed to decode. expected:[%+v] but got:[%+v]", expected, got)
+				}
+			}
+		})
+	}
+}
+
+func Benchmark_Decode_SlowReader_GoJson(b *testing.B) {
+	var expected LargePayload
+	if err := json.Unmarshal(LargeFixture, &expected); err != nil {
+		b.Fatal(err)
+	}
+	for _, chunkSize := range []int{16384, 4096, 1024, 256, 64} {
+		b.Run(fmt.Sprintf("chunksize %v", chunkSize), func(b *testing.B) {
+			b.ReportAllocs()
+			for i := 0; i < b.N; i++ {
+				index = 0
+				var got LargePayload
+				if err := gojson.NewDecoder(slowReader{chunkSize: chunkSize}).Decode(&got); err != nil {
+					b.Fatal(err)
+				}
+				if !reflect.DeepEqual(expected, got) {
+					b.Fatalf("failed to decode. expected:[%+v] but got:[%+v]", expected, got)
+				}
+			}
+		})
+	}
+}
+
+type slowReader struct {
+	chunkSize int
+}
+
+var index int
+
+func (s slowReader) Read(p []byte) (n int, err error) {
+	smallBuf := make([]byte, Min(s.chunkSize, len(p)))
+	x := bytes.NewReader(LargeFixture)
+	n, err = x.ReadAt(smallBuf, int64(index))
+	index += n
+	copy(p, smallBuf)
+	return
+}
+
+func Min(x, y int) int {
+	if x < y {
+		return x
+	}
+	return y
+}
diff --git a/decode_stream.go b/decode_stream.go
index b1c99f9..d7fc08a 100644
--- a/decode_stream.go
+++ b/decode_stream.go
@@ -17,6 +17,7 @@ type stream struct {
 	r                     io.Reader
 	offset                int64
 	cursor                int64
+	filledBuffer          bool
 	allRead               bool
 	useNumber             bool
 	disallowUnknownFields bool
@@ -52,6 +53,11 @@ func (s *stream) stat() ([]byte, int64, unsafe.Pointer) {
 	return s.buf, s.cursor, (*sliceHeader)(unsafe.Pointer(&s.buf)).data
 }
 
+func (s *stream) statForRetry() ([]byte, int64, unsafe.Pointer) {
+	s.cursor-- // for retry ( because caller progress cursor position in each loop )
+	return s.buf, s.cursor, (*sliceHeader)(unsafe.Pointer(&s.buf)).data
+}
+
 func (s *stream) reset() {
 	s.offset += s.cursor
 	s.buf = s.buf[s.cursor:]
@@ -60,10 +66,12 @@ func (s *stream) reset() {
 }
 
 func (s *stream) readBuf() []byte {
-	s.bufSize *= 2
-	remainBuf := s.buf
-	s.buf = make([]byte, s.bufSize)
-	copy(s.buf, remainBuf)
+	if s.filledBuffer {
+		s.bufSize *= 2
+		remainBuf := s.buf
+		s.buf = make([]byte, s.bufSize)
+		copy(s.buf, remainBuf)
+	}
 	return s.buf[s.cursor:]
 }
 
@@ -76,6 +84,11 @@ func (s *stream) read() bool {
 	buf[last] = nul
 	n, err := s.r.Read(buf[:last])
 	s.length = s.cursor + int64(n)
+	if n == last {
+		s.filledBuffer = true
+	} else {
+		s.filledBuffer = false
+	}
 	if err == io.EOF {
 		s.allRead = true
 	} else if err != nil {
@@ -131,8 +144,7 @@ func (s *stream) skipObject(depth int64) error {
 					if char(p, cursor) == nul {
 						s.cursor = cursor
 						if s.read() {
-							s.cursor-- // for retry current character
-							_, cursor, p = s.stat()
+							_, cursor, p = s.statForRetry()
 							continue
 						}
 						return errUnexpectedEndOfJSON("string of object", cursor)
@@ -142,8 +154,7 @@ func (s *stream) skipObject(depth int64) error {
 				case nul:
 					s.cursor = cursor
 					if s.read() {
-						s.cursor-- // for retry current character
-						_, cursor, p = s.stat()
+						_, cursor, p = s.statForRetry()
 						continue
 					}
 					return errUnexpectedEndOfJSON("string of object", cursor)
@@ -196,8 +207,7 @@ func (s *stream) skipArray(depth int64) error {
 					if char(p, cursor) == nul {
 						s.cursor = cursor
 						if s.read() {
-							s.cursor-- // for retry current character
-							_, cursor, p = s.stat()
+							_, cursor, p = s.statForRetry()
 							continue
 						}
 						return errUnexpectedEndOfJSON("string of object", cursor)
@@ -207,8 +217,7 @@ func (s *stream) skipArray(depth int64) error {
 				case nul:
 					s.cursor = cursor
 					if s.read() {
-						s.cursor-- // for retry current character
-						_, cursor, p = s.stat()
+						_, cursor, p = s.statForRetry()
 						continue
 					}
 					return errUnexpectedEndOfJSON("string of object", cursor)
@@ -256,8 +265,7 @@ func (s *stream) skipValue(depth int64) error {
 					if char(p, cursor) == nul {
 						s.cursor = cursor
 						if s.read() {
-							s.cursor-- // for retry current character
-							_, cursor, p = s.stat()
+							_, cursor, p = s.statForRetry()
 							continue
 						}
 						return errUnexpectedEndOfJSON("value of string", s.totalOffset())
@@ -268,8 +276,7 @@ func (s *stream) skipValue(depth int64) error {
 				case nul:
 					s.cursor = cursor
 					if s.read() {
-						s.cursor-- // for retry current character
-						_, cursor, p = s.stat()
+						_, cursor, p = s.statForRetry()
 						continue
 					}
 					return errUnexpectedEndOfJSON("value of string", s.totalOffset())
diff --git a/decode_struct.go b/decode_struct.go
index 4c79d6f..da32d6f 100644
--- a/decode_struct.go
+++ b/decode_struct.go
@@ -360,14 +360,14 @@ func decodeKeyByBitmapUint8Stream(d *structDecoder, s *stream) (*structFieldSet,
 									if !s.read() {
 										return nil, "", errUnexpectedEndOfJSON("string", s.totalOffset())
 									}
-									buf, cursor, p = s.stat()
+									buf, cursor, p = s.statForRetry()
 								}
 							case nul:
 								s.cursor = cursor
 								if !s.read() {
 									return nil, "", errUnexpectedEndOfJSON("string", s.totalOffset())
 								}
-								buf, cursor, p = s.stat()
+								buf, cursor, p = s.statForRetry()
 							}
 						}
 					}
@@ -457,14 +457,14 @@ func decodeKeyByBitmapUint16Stream(d *structDecoder, s *stream) (*structFieldSet
 									if !s.read() {
 										return nil, "", errUnexpectedEndOfJSON("string", s.totalOffset())
 									}
-									buf, cursor, p = s.stat()
+									buf, cursor, p = s.statForRetry()
 								}
 							case nul:
 								s.cursor = cursor
 								if !s.read() {
 									return nil, "", errUnexpectedEndOfJSON("string", s.totalOffset())
 								}
-								buf, cursor, p = s.stat()
+								buf, cursor, p = s.statForRetry()
 							}
 						}
 					}