I have been using Debian for several years and have just upgraded to Bullseye and I wish I hadn't. It has some bug fixes in it (which were more like minor inconveniences anyway), but Bullseye is SLOW. Its getting to be as bad as Windows and takes forever to start up.
Libreoffice is particularly slow, and some days I see a delay between hitting the keyboard and the results appearing on the screen. None of this was a problem with earlier releases. So right now I'm pretty pissed off with Debian
vmstat(8) and top(8) may be used to track that.
Pay attention on Cpu% "sy" in top, it shouldn't be high. New kernel -- new drivers. You may have hardware compatibility problems (or interrupt storm).
Also, check your drive health with smartmontools (bad drive may cause bad performance):
You can also measure its performance with fio