Interrrupt Latency means the time taken by the processor to

pass the control to ISR after the interrupt is raised.

Certainly this is the hardware feature.

Footprint of the OS matters because with the same compiler

and same optimization techniques, Different OS's will have

different footprint.

So this is can be looked upon while selecting the RTOS.

RTOS can be chosen looking at various API support it

provides. Synchronization support, Scheduler algos, and

memory management of the OS.