screenshots take 3-4 seconds to arrive in their folder

You might be running into an intentional behavior change with screenshots in newer versions of macOS. When you take a screenshot now, by default it pops up a little thumbnail of the screenshot in the lower right of your screen that lets you edit the screenshot before it's written to disk. Even if you ignore that, the screenshot doesn't get written until the thumbnail goes away. You can disable this in Screenshot.app > Options > Show Floating Thumbnail, and then your screenshots should get written and show up immediately. Hopefully that's your issue and not just Finder failing to update.

switching User accounts takes ages

MacOS has to wait for all your user services to stop and then start all the next user's services when switching, so it might be worth looking into what services are attached to your users and trimming them down via launchctl or something.

I'm typing this on a 12 year old macbook that's still perfectly snappy, so hopefully you can breath some life into yours. 3 years certainly shouldn't be a death sentence.