[SOLUTION FOUND!] FPS DEGRADATION & STUTTER OVER EXTENDED PERIOD OF GAMEPLAY

This thing has pasted me since 7th of January. I've had to clear the %AppData%-cache and restart the game every few hours. Sometimes it happens more often even. This has pested AMD CPU (3950X) + nVidia (4080) systems at least.

Now another solution has occurred, which is also the key for GGG to solving this permanently.

Just type /clear

FPS 47 in base pre-/clear
FPS 120 (capped on 4k) post-/clear