Small brushed DC motor speed falls off above a certain PWM frequency?
Thanks for teaching me an English word new to me.
Judder A spasmodic shaking.  (like Jitter &amp; Shudder combined)
No-load RPM is voltage-controlled (i.e. avg. Vdc = %PWM)  and Torque is current-controlled and visa-versa for accleration or braking.
The coil commutation converts the DC into AC to provide continuous torque in one direction but depends on magnet positions relative to the coils.
The magnetic commutation frequency and the PWM commutation frequency act with a nonlinear mixing effect on Torque.  The effect is a modulation of rotational torque. Each frequency has harmonics from the discontinuity and the result is Intermodulation or Aliasing Effect.
I would expect maximum Judder Effects when the PWM and frequency approach synchronous cutoff/on of alternate phases with each PWM cycle and then torque smoothing yet weakening above this PWM rate.
There will be some flying effect with increasing speed or rotational inertia and smoothing effect with increasing PWM, but it also increases the impedance of the motor from PWM with X(f)= 2πfL, yet L is also changing with rotor angle from magnet strength.
Thus when PWM f gets too high, it becomes current-limited for torque and when the alias frequencies of harmonics get too low, the torque gets more jerky as it changes speed and thus spastically &quot;judders&quot; more.  I noticed this before as well on BLDC fan motors and opted to go with linear Vdc controlled cooling due to the erratic noise.