Terrible (CPU) optimization in end game content

I have made a test to check whether the performance issues are actually unique to end game content, since alongside my progression I noticed the performance getting worse and worse as I went, coming to a point where it's literally unplayable (below 40fps with stutters).

I conducted the test using the same character (Mercenary), using the same skills which I used the same amount of times on an almost exact same amount of enemies on screen in two different parts of the game. One in act 1, and the other on end game maps. I attracted around 15 enemies to myself and proceeded to drop 4 gas grenades and detonate them with 2 explosive grenade. Surely there can't be such a big difference? Well...

Act 1:
No enemies on screen: 110 fps
With enemies on screen: 85 fps
During the explosion and few seconds after detonating my grenades: 60 fps

End game maps:
No enemies on screen: 80 fps
With enemies on screen: 65 fps
During the explosion and few seconds after detonating my grenades: 40 fps

I used all the default graphics settings, I had no other programs running on my computer, and I tested it both on Vulkan and Directx12, both had extremely similar results.