That’s quite odd. Blizzard did change some engine things in 9.0.5 which caused problems but “low power” on M1 is kind of odd - curious if it’s actually causing M1 to go into low power mode or just 9.0.5 FPS drops + label change make it look like so.
With macOS there is the problem that there are no good monitoring/benchmarking tools so even if I got one for testing it would be hard to tell what’s going on there.
Can you run some other game of similar requirements and compare like temperatures or power draw between it and WoW?