satyabrata:
Higher baud rate (115200) is not my problem..Lower baud rate is my problem, motor slows down at lower baud rate and I don't want that.
Your call to Serial.println is going to cause a delay. The library can handle (buffer) around 32 bytes, from memory, after which it blocks (waits). At high baud rates it can clear the buffer without blocking. At low baud rates it won't. Your observed behaviour is totally explained by that.