Severe stutter/low frame rate with SuperpositionBenchmark on RTX-2080TI

I was getting about 70 fps running the Superposition benchmark at 4K which should be expected for a 2080-Ti. At some point after upgrading from at TR-2990wx to TR-3970 with a new motherboard and upgrading from Mint 19.3 to Mint 20 I see tremendous stutter and low frame rate, although the card works displaying static images on a 4K monitor.

I booted from a SSD containing a fresh Mint 19.3 install using the nvidia-driver-440 package, but saw the same problem. A windows 10 install from another SSD resulted in normal, high frame rates, so this should not be a hardware problem with a broken card.

I have tried drivers from nvidia-driver-435 to nvidia-driver-450 without success. I have tried other unigine benchmarks which also show a new (poor) behavior. I am not sure how to proceed, but this is very frustrating. My GPU is now crippled.