Nothing with interrupt handlers is fully deterministic. Deterministic
timing means exact to the clock pulse, the same every time.
Ok, if an interrupt comes in during a 2-cycle operation (and I think the AVR has few of them), then the instruction is going to complete. So there is (at 16mhz) 62.5ns of jitter.
Real-time means guaranteed response within some specified time limit.
LinuxCNC runs on a real-time version of Linux. The response time
depends on the speed of the hardware, but remember the hardware is
1000 times faster than a microcontroller, so a few dozen us is typical.That's not much more than the jitter caused by the timer0 interrupt on
an Arduino.
Well, it doesn't make much sense to compare latency with jitter. And latency is unimportant for a timer-generated interrupt. But I suspect if you compared jitter/latency on an AVR to jitter/latency on RTLinux you would find the AVR 'wins'.
Of course, for CNC use, it really doesn't matter. I use DOS-based TurboCNC for my CNC needs, and what I like about it is...low latency. 8)
I turn on the computer, it boots from a compact flash card, and about 18 seconds after turning it on it is ready to go!