Ideas
Captured some benchmarks that show this.
normal.jpg shows a normal result in this bench mark map (I use the same map, stand in the same place, with all the same graphics settings, nothing has changed between the runs which are 3 minutes).
spikes.jpg shows a 3 minute run where I do a few examples of the glass bug, that's what the three spikes are, followed by the final section where the problem is even worse than the previous examples as I also move the guns & fire them while inside the glass. I have seen that sometimes the glass has to continue to be shot so the "cracked glass" effect is all over the piece, and it really kills the FPS when you point the gun inside and/or shoot with that cracked effect there. Not that glass never goes away unless the entire building piece is destroyed so I don't know if windows and broken pieces are a local physics effect, or if this might impact the entire server.
worst.jpg is a cut of that final extreme drop, showing that the FPS has dropped down as far as 33 fps, which is around 5 times lower than it normally is.