I've got Guitar Pro 5 for my old PC ( I got a Mac as well but the PC is in my room with my guitars).

When the RSE CPU goes above 60% the sound begins to break and doesn't sound good at all. This does not happen on the Mac, is it because it's a better computer so the CPU doesn't get that high.

Do you know what's wrong and what I could do to fix it?
That happens to me too, but I just close out some of the running programs so that it doesn't slow GP down. That seems to work for me
Yeah, I just discovered that you just gotta change it to midi...

But is there a way so it does this by default, so I don't have to do it manually all the time?
the rse engine used to lag on my old computer, but i don't have any problems on my latest build.

i reckon it's mostly cpu dependant, and macs are known to use their processing power a lot more efficiently than windows based systems.