Did you check the temperature of the cpu and gpu? It might be a overheating problem that causes the fps drops.

Why do you consider AppImages as last resort?

I can understand that in a distro the main repo need to be prioritized to avoid bloat of repetitive dependencies that could happen with a lot of AppImages.

I can’t understand why so many people are opposed to it as an supportive role from a practical and complementary perspective.

Canonical sure know how to name them.