Baud rate change slows down void loop and stepper motor speed??

Other Hardware LEDs and Multiplexing
13

And you have not understood my suggestion.

You appear to be calling serial.print() tens of 1000s of times a second (every time loop() is executed), the poor serial port is overwhelmed. Do you get that?

I have not modified the standard library apart from Serial.println(_currentPos);

That's may seem like a small one-line change but it has a lot of ramifications.

Try this

void loop() {
   if ((millis() % 1000) == 0)  stepper.currentPosition(); // this function retuns current stepper position over serial.println()...written in accelstepper library

That should only print the position every second (it may miss a beat every now and then). I don't know if that's the problem but it can't do any harm.

Rob