A much bigger snail pace than before is most accurate.

Benchmark both cpu and gpu. It might be one of them failing. Check temperatures with the benchmark as well. Also test memory. I’m going to assume it’s throttling or your gpu is having issues. CPU would be obvious because it wouldn’t just be gaming that’s messing up. Normally if it’s only under heavy load it’s because of high usage applying a stress somewhere that’s forcing symptoms of throttling etc.