Some would say compatibilty but they go hand-in-hand so I'd say both. It's very game/issue-specific though. Some games/systems will have huge bumps from 7.x to 10.x but others none at all... just depends on why they didn't have great performance to begin with.

These days Proton is in a good spot so it's a lot of weird outliers (like the codec thing) that weren't necessarily showstoppers, or support brand new games need which is why they may seem more significantly impacted.