From AccelStepper.h:
"/// \par Performance
/// The fastest motor speed that can be reliably supported is about 4000 steps per
/// second at a clock frequency of 16 MHz on Arduino such as Uno etc.
/// Faster processors can support faster stepping speeds.
/// However, any speed less than that
/// down to very slow speeds (much less than one per second) are also supported,
/// provided the run() function is called frequently enough to step the motor
/// whenever required for the speed set.
/// Calling setAcceleration() is expensive,
/// since it requires a square root to be calculated".
Which Arduino?