Please provide the following when creating the Ticket:

Game Version number: 1.3.0.157
Modded?: (Yes/No) No. Happens even when not running any mods.
If modded, please list the mods:

Issue Description:
Tested and happens in Conflict (Full game or one of the smaller ones), plus gamemaster.
Happens on local-only game, and also on public servers or private servers.
Happens with and without mods, although kicks in quicker when running mods or on a busy server.

When opening the inventory screen, either by searching a corpse for items, or using the arsenal, or just tabbing into the inventory screen, the game suddenly performance drops rapidly, often permanently with FPS dropping to 15,10 and sometimes down to 2 FPS, rendering the game unplayable.
Sometimes you can recover by going into windowed mode and back again (alt/return), but usually it is fatal and the only way to recover is to exit the match and re-enter.

I can however, run around in game all day with good performance (60+ FPS) if I am careful to never use the inventory.

This seems to have started with 1.3, on both Experimental and now the Main branch.

Happens with or without addons.

Lowering the render scale to <60% sometimes helps recover as well.