[cronokirby]

Sleeping just long enough to fill the remaining time-slot for an operation is actually a valid countermeasure; in theory.

The problem is figuring out how much sleep is necessary based on what variable time work has been done, and actually sleeping the right amount. This can be difficult with the granularity involved.

In practice, attempting to fill the remaining time is either not effective, or requires a level of instrumentation that severely degrades performances.

I'd recommend section 5 of https://eprint.iacr.org/2016/613.pdf for a survey of these ideas.