Yeah, the external timer behaviour is fairly strong evidence. Curious though. These cams all seem to have a milli- and micro-second hw clock, and can both schedule and sleep against either. But it's also true that every cam has some weird quirks. And I don't know the 5d2 internals well.

From what I've seen, the image capture process is state machine based and tries to avoid sleeps and delays. Which makes sense for RTOS and professional photography.

If you care enough to debug it, pop into the discord and I can make you some tests to run.