I have some code where interrupt jitter is deal killer. There are just one interrupt source and that need to be as precise as possible. For ATMEGA the max interrupt response is the length of current instruction + call to interrupt routine.
But my code behave erratic. The interrupt delay seems much larger. Finally I find where trouble lies. The GCC disables the interrupt by itself!!! So here is simple example of that. No interrupt usage at all in source code.