[treetrouble]

It's not something that's easily perceptible unto itself unless you get into very radical inconsistencies. As an example using arbitrary numbers, a quarter note would be 500 ticks one time and 493 ticks the next and so on...

It can be measured by recording the audio on a (timing accurate!) dedicated system and analyzing the waveform

Subtle as it may be, timing accuracy has an essential impact on how we perceive music as a whole. This is one contributor among a few as to why vintage drum machines and sequencers have become so sought after as software-baed music production has become more prevalent. The Akai MPC series (at least the 2000XL and previous) models have been found to have the most consistent timing of any machine with the Atari ST being the strongest computer sequencer.

Sorry, I don't have time to cite this stuff (breaking my own rule) but it should be easy to google

As an aside, poor timing accuracy can be a reason why in pulse-based music that some human musicians "just suck" even if they're otherwise proficient. Or some musicians are "just have it" even though they're playing something unremarkable