Arduino Zero External Interrupt Latency

I am trying to attach an external oscillator to an interrupt pin.

The oscillator is to act as a reference clock/timing standard, because practice has shown that the internal clock(s) are not accurate enough, due to the use of resonators in the processor clock circuit.

I have tried various AVR Arduinos but the latency in handling the ISRs is a problem.
I have even tried a ESP32, which was even worse.

I was looking around the internet and found a YouTube video were the person was using an Arduino Zero in a similar way to generate a more reliable clock, but at a lower frequency than I can get away with.

I have an Arduino Zero in another project that is in use most of the time, however I am now considering giving it a try. Before dismantling this project I wondered if anyone has an actual figure for the number of program cycles and/or the max input frequency the Zero ISR will handle reliably.

But... the ESP and SAMD boards all have crystals for timing, so perhaps they are accurate enough without your external oscillator.