gogo jsonpb is 10x slower than golang json Unmarshal?

[blackbeans]

Go version:1.13
protobuf: 3.6.x

code :

message Cursor {
    string ID = 1 [json_name = "id",(gogoproto.jsontag)="id"]; 
    uint32 Type = 2 [json_name = "type",(gogoproto.jsontag)="type",(gogoproto.customtype)="uint16",(gogoproto.nullable) = false]; 
    int64 Offset = 3 [json_name = "offset",(gogoproto.jsontag)="offset",(gogoproto.nullable) = false,(gogoproto.customtype) ="encoding/json.Number"]; 
    int64 QTimestamp = 4 [json_name = "timestamp",(gogoproto.jsontag)="timestamp",(gogoproto.nullable) = false,(gogoproto.customtype) ="encoding/json.Number"];
    int64 EndOffset = 5 [json_name = "end_offset",(gogoproto.jsontag)="end_offset",(gogoproto.nullable) = false,(gogoproto.customtype) ="encoding/json.Number"]; 
}

func BenchmarkJsonPb(t *testing.B) {
	rawMark := []byte(`{"id":"1224","type":2,"offset":9136,"timestamp":1593935873,"end_offset":0}`)
	unmarshaler := &jsonpb.Unmarshaler{AllowUnknownFields: true}
	for i := 0; i < t.N; i++ {
		var cursor Cursor
		unmarshaler.Unmarshal(bytes.NewReader(rawMark), &cursor)
	}
}

func BenchmarkJson(t *testing.B) {
	rawMark := []byte(`{"id":"1224","type":2,"offset":9136,"timestamp":1593935873,"end_offset":0}`)

	for i := 0; i < t.N; i++ {
		var cursor Cursor
		json.Unmarshal(rawMark, &cursor)
	}
}

goos: darwin
goarch: amd64
BenchmarkJsonPb-4         104812             11316 ns/op
BenchmarkJson-4           671342              1805 ns/op

above all, i have upgrade our project to protobuf and used jsonpb to output json format for communication .But i found than jsonpb Unmarshals the same json-data is 10x slower than golang 1.13.x .

i lost my way~