How to check maximum frequency generation capability PWM of microcontroller?
In general terms, the fastest PWM speed available depends on which clock the PWM hardware peripheral is inheriting, and which divisor options the PWM peripheral got. Some PWM peripherals might be able to work at system clock, others at raw external oscillator clock, others system clock divided by something. 

Many MCUs also have multiple available PWM resolutions (8, 16, 32 bits), which may affect speed.

And finally the port I/O will have an activation delay, we are talking nanoseconds there.