fps bug

I'm using an RTX 4080 Super.
When enemies increase in endgame content, my FPS often drops from 120 to around 40-60. Can this be fixed?

Also,
If you die to a boss, the map resets and only one monster remains.

Is it intended that using Sigil of Power lowers spell power?
You can check this in the menu after using Sigil of Power.
Using other skills will restore it.
(Unique equipment)