Arduino/Android Bluetooth delay

Hello guys,
We already asked that question on stackoverflow but no answer. I hope you will find !

We are developping an app that uses Bluetooth library to communicate with an Arduino in bluetooth via an HC-05 module. We made a dummy configuration to test the delay without any computation from eather the Arduino or the app and we have a huge delay of about 1 second between a request and an answer...

Protocol looks easy : Android send byte -2 and if byte received is -2, Arduino send -6, -9 and Android answer again and again.

Android Code :

h = new Handler() {
            public void handleMessage(android.os.Message msg) {
                switch (msg.what) {
                    case RECIEVE_MESSAGE:                                                   // if receive massage
                        byte[] readBuf = (byte[]) msg.obj;

                        for(int i=0;i < readBuf.length;i++)
                        {
                            if((int) readBuf[i] != 0) {
                                txtArduino.append(String.valueOf((int) readBuf[i]) + ", ");
                            }
                        }
                        byte[] msg = {-2};
                        mConnectedThread.writeByte(msg);
                        break;
                }
            };
        };

Arduino Code :

const int receveidBuffLen = 8*4;


void setup() {
  Serial.begin(115200);
}

void loop() {
    if (Serial.available() > 0) 
    {
      byte buff[receveidBuffLen];
      Serial.readBytes(buff, receveidBuffLen);

      for(int i=0; i < receveidBuffLen;i++)
      {
        if(buff[i] == (byte) -2) // 254
        {
            byte message[2] = {(byte) -6, (byte) -9};
            Serial.write(message, 2);
            Serial.flush();
        }
      }
    }
    delay(3);
}

Does anyone know where the delay comes from?

We changed the HC05 baudrate (from 9600 to 115 200) : nothing happened. We changed HC05 with another : nothing happened. We used the Blue2Serial library (Bluetooth as SPP) before and delay was the same... We used another controler (ESP8266) and delay still was 1 second...

Thanks in advance !