Possibly pointing to Scheduler: First off, this is not the case (for me) on MSFS currently (with one exception, see below). But I know the effect (and the disbelief) from X-Plane, where it is very pronounced (forgot the numbers but the difference is big enough to decide between “no fun” and “playable” in dense areas) and there I had it first on the Rift S, then on the Reverb G1 - so the HMD wasn’t the culprit there.
Secondly, I get this effect in MSFS when my Reverb went to sleep and I reactivate it, then it sometimes ends up in some kind of upset state and the reduced, stuttery framerate recovers when I Alt+Tab MSFS out of focus. That sounds a lot like something ended up on the wrong CPU core.