PSA about the Steam client performance:
Turns out that in your Steam library, rare achievements are marked with an animated light effect. This animation somehow takes up an astounding amount of system resources especially on older PCs/laptops.
If you notice that your PC slows to a crawl whenever your steam library is open, you can go to Steam > Settings > Library and then turn on Low Performance Mode. It simply disables a few insignificant animations but the performance gains can be massive. On my old Dell Latitude, the CPU usage goes from >50% to <10% just from changing this one setting!
I have no idea why they put this under "Library" and not "Interface" cause I've been looking for a setting like this for months but I finally found out about this and now you have too!