Modern macos is pretty unusably slow on old apple hardware. The same hardware ran earlier versions of the OS just fine. I’m not sure what’s going wrong at apple, but I don’t think they have enough performance engineers working on the Mac.

I run a CPU monitor in my dash (and have for years). The efficiency cores are very often busy doing some rubbish tasks I didn’t ask for while my computer is idle. Things like reindexing files, or re-scanning my photos for faces or something like that. I’ve started to think of them as the CPU cores apple put in my computer to keep their useless internal projects from ruining their hardware.

Linux annoys me regularly, but at least my cores are almost totally idle whenever my hands aren’t on the keyboard.

Window server pegs CPU on older hardware doing any sort of trackpad gesture etc. Wasn't the case in Mojave and adjacent releases. Big Sur on are noticeably worse than the earlier releases. Disabling animations/transparency saves a bit, but it's still remarkably poor.