what causes certain games to severely drop fps for a few seconds?
Certain games such as the re4 remake and stray, will randomly drop to 5 fps for a few seconds, then jump back up to 30-40 fps. These games normally run fine at 40fps but I'm not sure what causes the drops.
You might try cryoutilities and see if it helps. Sometimes drops like that are due to the system having to reallocate RAM/swap or other system resources, causing a drop. A lot of the cryoutilites addons are meant to reduce the frequency and severity of those drops, so it might help.
Otherwise your best bet is to enable logging of your performance data through mangohud, and whenever you have a fps drop check the logs/charts and see what happened.
Maybe the long-standing SMT bug? Basically, when two threads on the same core are running, and one of the threads discards its cache, due to this bug, the cache is discarded for all the threads on the same core. This causes a temporary FPS dip.
A proper fix is due in SteamOS 3.5, but you can also turn off SMT by installing PowerTools Decky plugin to see if it helps.
The heat may be a cause. I’ve experienced it a few time during long session. Just let the SD cool for 10 min on sleep mode and you’re good for 1-2h, depending of the game.