Hi What is your delta-time? Is it a interval time of events?
I think, you would be measure time-span by the Timer
It is better rather than "interrupt fired with known clock frequency"
The PSoC/ARM M0 architecture allows Multi-interruptions,
Good coded interrupts will get work.
Just flip a pin at beginning and end of loop, and observe with a scope. Thats typically a low
Or use a free-running counter, and use its capture function.
you could start a timer at the beggining of the program with an apropiate clock source and leave it running then check its value inside the millis() function