transaction: don't recalculate size when decoding from buffer

name               old time/op    new time/op    delta
DecodeBinary-8       3.17µs ± 6%    3.14µs ± 5%     ~     (p=0.579 n=10+10)
DecodeJSON-8         12.8µs ± 3%    12.6µs ± 3%     ~     (p=0.105 n=10+10)
DecodeFromBytes-8    3.45µs ± 4%    2.73µs ± 2%  -20.70%  (p=0.000 n=10+9)

name               old alloc/op   new alloc/op   delta
DecodeBinary-8       1.82kB ± 0%    1.82kB ± 0%     ~     (all equal)
DecodeJSON-8         3.49kB ± 0%    3.49kB ± 0%     ~     (all equal)
DecodeFromBytes-8    1.82kB ± 0%    1.44kB ± 0%  -21.05%  (p=0.000 n=10+10)

name               old allocs/op  new allocs/op  delta
DecodeBinary-8         29.0 ± 0%      29.0 ± 0%     ~     (all equal)
DecodeJSON-8           58.0 ± 0%      58.0 ± 0%     ~     (all equal)
DecodeFromBytes-8      29.0 ± 0%      21.0 ± 0%  -27.59%  (p=0.000 n=10+10)

pkg/core/transaction/bench_test.go

@@ -0,0 +1,55 @@
+package transaction
+
+import (
+	"encoding/base64"
+	"testing"
+
+	"github.com/nspcc-dev/neo-go/pkg/io"
+	"github.com/stretchr/testify/require"
+)
+
+// Some typical transfer tx from mainnet.
+var (
+	benchTx     []byte
+	benchTxB64  = "AK9KzFu0P5gAAAAAAIjOEgAAAAAA7jAAAAGIDdjSt7aj2J+dktSobkC9j0/CJwEAWwsCAMLrCwwUtXfkIuockX9HAVMNeEuQMxMlYkMMFIgN2NK3tqPYn52S1KhuQL2PT8InFMAfDAh0cmFuc2ZlcgwUz3bii9AGLEpHjuNVYQETGfPPpNJBYn1bUjkBQgxAUiZNae4OTSu2EOGW+6fwslLIpVsczOAR9o6R796tFf2KG+nLzs709tCQ7NELZOQ7zUzfF19ADLvH/efNT4v9LygMIQNT96/wFdPSBO7NUI9Kpn9EffTRXsS6ZJ9PqRvbenijVEFW57Mn"
+	benchTxJSON []byte
+)
+
+func init() {
+	var err error
+	benchTx, err = base64.StdEncoding.DecodeString(benchTxB64)
+	if err != nil {
+		panic(err)
+	}
+	t, err := NewTransactionFromBytes(benchTx)
+	if err != nil {
+		panic(err)
+	}
+	benchTxJSON, err = t.MarshalJSON()
+	if err != nil {
+		panic(err)
+	}
+}
+
+func BenchmarkDecodeBinary(t *testing.B) {
+	for n := 0; n < t.N; n++ {
+		r := io.NewBinReaderFromBuf(benchTx)
+		tx := &Transaction{}
+		tx.DecodeBinary(r)
+		require.NoError(t, r.Err)
+	}
+}
+
+func BenchmarkDecodeJSON(t *testing.B) {
+	for n := 0; n < t.N; n++ {
+		tx := &Transaction{}
+		require.NoError(t, tx.UnmarshalJSON(benchTxJSON))
+	}
+}
+
+func BenchmarkDecodeFromBytes(t *testing.B) {
+	for n := 0; n < t.N; n++ {
+		_, err := NewTransactionFromBytes(benchTx)
+		require.NoError(t, err)
+	}
+}

pkg/core/transaction/transaction.go

@@ -143,8 +143,7 @@ func (t *Transaction) decodeHashableFields(br *io.BinReader) {
 	}
 }
 
-// DecodeBinary implements Serializable interface.
+func (t *Transaction) decodeBinaryNoSize(br *io.BinReader) {
-func (t *Transaction) DecodeBinary(br *io.BinReader) {
 	t.decodeHashableFields(br)
 	if br.Err != nil {
 		return
@@ -159,6 +158,14 @@ func (t *Transaction) DecodeBinary(br *io.BinReader) {
 	// to do it anymore.
 	if br.Err == nil {
 		br.Err = t.createHash()
+	}
+}
+
+// DecodeBinary implements Serializable interface.
+func (t *Transaction) DecodeBinary(br *io.BinReader) {
+	t.decodeBinaryNoSize(br)
+
+	if br.Err == nil {
 		_ = t.Size()
 	}
 }
@@ -240,7 +247,7 @@ func (t *Transaction) Bytes() []byte {
 func NewTransactionFromBytes(b []byte) (*Transaction, error) {
 	tx := &Transaction{}
 	r := io.NewBinReaderFromBuf(b)
-	tx.DecodeBinary(r)
+	tx.decodeBinaryNoSize(r)
 	if r.Err != nil {
 		return nil, r.Err
 	}