Some additional information for the devs on locating the source of this FPS drop which I'm also experiencing:
A: It worsens the moment someone spawns in your proximity (was absolutely reproducible with two buddies of mine).
B: Using the tip from the forum member "Droopy Piles" who stated:
"I CAN CONFIRM THIS WORKS!!!
there is a .cfg data - open it and run it with editor.
scroll down till u find this and change both parameters to 3:"
GPU_DetectedFramesAhead=3; " "
When applied, this seems to fix the problem, but really just delays it. You won't have an FPS drop for a bit, maybe 15 mins, but that's no definite time, and then it starts again just like without the fix.
Hope this helps.