I would start checking your CPU. Just look up older test results and verify in some benchmarks that it gives the performance it should.
You won't see single thread limitations that easily. Might be that overall CPU-Usage is 10%, but it's already limiting on a single thread.

Knowing your ...

this is an issue which has been long standing for me but ignorable until recently. in vanilla (with some qol mods), this issue would happen every five minutes or so, and would constitute a brief drop to 45 fps, then go back up to 60. i noticed it only because i always have my fps/ups counter on. i ...