Why does classic tax my system more than retail?

This has been an issue for a few months now. I play on 3 different computers and all seem to be taxed more by classic than retail. My desktop, the GPU fan sounds like a dag gom jet engine when it’s being heavily used, and every time i play classic it does this right away. On retail though, it will only get angry sometimes. Emerald dream, sometimes in Valdraken… but for the most part it doesn’t have trouble.

No I know what you’re going to say… “Jyxi, did you try turning your addons off?”
Yes. Yes I did. And it still sounds like we’re taking off! I even have tried lowering the graphic settings.

Anyone have a similar experience?