I am not able to fully reproduce this issue easily as it is an issue that occurs in the latter stages of a characters life span.

Having found an assault vest and utility buttpack to add the final and highest available slot space for my character (42 slot top, 30 slot pants, 90 slot field backpack, 60 slot assault vest with buttpack attached) i found that once my inventory was totally full and i had both gun slots on back filled with an extra gun in hand, i would experience consistent slight frame issues, as if running on a rig that couldnt quite hack it, however when then opening doors for the brief animation this is happening everything runs smooth as it should for about half a second

After a little more testing i then discovered that holding a melee item in hand with 2 guns on back completely negated any performance perceived hit and game was running 100 smoth frames (i run on a 144hz monitor with gsync on and vsync in game off so smooth is particularly noticeably different from when there is any kind of frame lag)

It is tough to reproduce the issue on multiple servers as it typically has taken me a number of days to get that geared on one server where the issue begins to occur. Once it did start happening, i searched the issue and found this could be to do with the fact all items are rendered in game in 3d in full even when in your characters storage inventory slots, i began to test and found if i dropped some items the issue would lessen.

Maybe something worth looking iinto as when you are at that point you have that much loot from a run to say NWAF, the literal last thing you need is frame issues in the event of a firefight