Steppers cannot accelerate instantly - particularly here as you have significant
driven mass.
AccelStepper, not Stepper, is the library to use, as you can set the max speed and max
acceleration.
If you set the acceleration conservatively then you can experiment to find the
max speed for reliable operation, then up the acceleration until it miss-steps and
back-off 20% or so.
Miss-stepping can be caused by overload (the motor cannot pull hard enough
for the load / acceleration), or by resonance. Mechanical load will both
affect the resonant frequency and add some damping, so you can only tune
a stepper under load. Micro-stepping drivers reduce the amplitude of resonance.