Max step pulse frequency from ATmega2560

Hi,
I have been looking around a lot to try to find out what the max frequency of pulses the ATMega2560 can produce to drive each of the step driver axes in a Marlin 2, ATMega2560, Ramps1.4, X,Y,Z,E type set up is.
I have seen a few discussions, with a few different figures given.
Is the stepper interrupt set to 10kHz - ie it can give 1 step pulse for each of the 4 axes at a max frequency of 10kHz ?
I have seen 100kHz mentioned, split between all of the axes ?
Now, i am not asking how to get my axis moving at whatever speed, so forget any step motor / driving limitations
If you were to have a 200 pulses per rev motor, attached to a 5mm per turn leadscrew, no micro stepping, then 10000 steps /sec would give a max travel rate of 250mm/sec.
In the config.h file for Marlin 2, the default feedrate is set to 300 for X & Y ???
So i guess that 10kHz is not the max frequency ?
I have also seen that it could output upto 4 pulses for each interrupt, but that these pulses would not be percisely synchronized - so i guess not much use for a 3D printer ?
So, can anyone tell me what the max frequency to operae all 4 axes synchronously is please ?
Thanks