I haven't seen this one reported anywhere, so it could be something wrong with my setup; but I'm pretty sure it started for me with 4.0 patch (I'm using the GOG version).
When I open the inventory while having weapons equipped and put the cursor over any other weapon, the comparison tooltips show up as normal. Except in my case, letting them staying opened like this for several seconds ALWAYS leads to a brief "freeze" or "lag" that continues to repeat itself pretty much endlessly until I stop comparing stuff. It happens regardless of graphics settings, and I'm not sure it's even videocard related, since I'm seeing CPU activity "dip" when that happens as well.
The game runs well enough for me otherwise, but I'm putting this out there just in case you'll want to investigate this further. DxDiag attached.
Here's a short video which probably better describes the issue than I do:
This is with every in-game setting being on default.
Comparing other items doesn't seem to trigger this behavior, but comparing weapons always does.
Switching to other set before reproducing the bug is to show that it's not the skull sabre lighting effects which cause this. Literally no option in graphics settings affects this behavior. Even if I cap framerate to 30, I'm still getting those drops (to ~20 FPS in that case).
Thanks for reading.