This has to do with 3D, specifically, as it works perfectly fine in 2D (actually it affects some aspects of 2D, as I later found out). In 3D it hangs for a while (like up to half a minute) when I create 3D nodes or add certain properties (like a mesh material or a collision shape) for the first time. So after creating one spatial and waiting, I can then create as many spatials I want and it doesn't hang, and the same thing goes for other nodes and the properties. Except if I restart Godot, then it hangs the first time I select each of the nodes I previously created.
It also takes a longer while to load any 3D projects, or scenes with a 3D root node, and also any new projects (because it defaults to the 3D view, I suppose), and when I press F5 it also takes a while to run the 3D projects (every time).
Again in 2D nothing of this applies. It loads, runs, creates nodes, etc, all nearly instantly.
I don't see anyone else talking about this, so I'm not sure this is a problem with my system (which is indeed kinda old and screwed).