The smallest period of Interval Timer

Hello all!

I created project, which include Interval timer, UART and several PIO.

I defined interrupt handler function for interval timer and for UART. I didn't set priorities for interrupt handlers.

I set Fixed period of Interval Timer in range between 5 up to 30 usec.

so I saw:

if the Interrupt Timer period is less then 20 usec - program can't proccess every interrupt. How to define the smallest period for Interval Timer - I want program performes every interval timer interval and I don't want to use priorities. If it depends on timer processing of one operation , how can I define this time?

Thank you

P.S.

I'm sorry if you decide my question is foolish..