Last week I reinstalled all my 6800 GPU drivers after having the card for a month. Everything was running great on default GPU settings, temps were 75-80 while playing No Man's Sky and Hell Let Loose, GPU usage was constant at 99%.
However, since a few days ago I basically lost that performance. GPU usage jumps between 30 and 60%, CPU is usually <20%, and GPU temps are around 55C and 60C junction. This is some massive underperformance that I can't find an explanation for. No Man's Sky is not going above 60/70fps, on Ultra Settings or Low Settings, on 1440p or 720p. Assassin's Creed Valhala, for example, runs with GPU at 99% and 120fps. 3D Mark Time Spy runs at 99% and 80+C junction. I heard people considering CPU bottlenecking but that wouldn't explain why everything worked in the past or why changing graphics settings has no effect on the fps.
Another thing about the games where the card is underperforming (No Man's Sky and Hell Let Loose) is that on the menus I get 144fps with 97+% GPU usage and 80C junction, but as soon as I load/start the game the performance is gone, the card's temp lowers to 60/62 junction, and the GPU usage to 60%. CPU never goes above 20% or so, from what I remember.
The only software I installed in the last few days was the Asus AI Noise Cancellation and Argus Monitor to set a fan's speed based on GPU temps. I have since removed both and the issues remain.
GPU: MSI Gaming X Trio 6800, stock
CPU: 5600x, stock (Resizable BAR enabled)
RAM: 32GB @ 3200Mhz
Disk: WD SSD NVMe