Not tweaking device parameters that much, but adding devices, moving devices, deleting devices (even if the track they're on doesn't have any clips), changing patches on some VST instruments, starting clips, changing clips, turning record on, off... ...sometimes even without me doing anything. And this in sets with moderate CPU usage (20 - 35%). A few times I've seen the CPU meter peak briefly when audio gets interrupted, but usually, it doesn't and neither does the disk overload indicator ever light up.
I realise this is a beta, but I find it really annoying and a bit worrying to be frank - I sure hope my two months old MBP will be able to run L8 smoothly, when it's finally released! I should add that L7.14 runs fine on the same machine.