Stepper motor speed limited

[NickAmSee]

I'm using a ULN2003 driver with 28BYJ-48 stepper motor on an Arduino Mega. And the following code:

#include <Stepper.h> // Include the header file
#define STEPS 2038
Stepper stepper(STEPS, 8, 10, 9, 11);
void setup() {
}
void loop() {
stepper.setSpeed(18);
stepper.step(-2038);
delay(200);

}

Setting speed values higher than 18 only results in beep sound of the motor. What is the reason for this slow speed?

[JCA34F]

Because of the high gear ratio (64 to 1), the highest RPM I've gotten is about 13 RPM without acceleration. With the AccelStepper library I was able to get about 24 RPM (820 steps per second). Install the AccelStepper lib from the Arduino IDE library manager and try it.
PS For the 28BYJ-48, steps per second = RPM * 2048 / 60.

// Bounce.ino
// -*- mode: C++ -*-
//
// Make a single stepper bounce from one limit to another
//
// Copyright (C) 2012 Mike McCauley
// $Id: Random.pde,v 1.1 2011/01/05 01:51:01 mikem Exp mikem $

#include <AccelStepper.h>

// Define a stepper and the pins it will use
AccelStepper stepper(4, 8, 10, 9, 11);

void setup()
{  
  // Change these to suit your stepper if you want
  stepper.setMaxSpeed(820);
  stepper.setAcceleration(1000);
  stepper.moveTo(2048);
}

void loop()
{
    // If at the end of travel go to the other end
    if (stepper.distanceToGo() == 0)
      stepper.moveTo(-stepper.currentPosition());

    stepper.run();
}