Hi, unfortunately the problem starts again. I assume it can only be a hardware problem then? Or could the Mac be too old to run with about 420GB (on a 500GB SSD)?
420 of 500? That’s about 85%. Not enough free space!
I should have been more emphatic when I suggested getting a 1-terabyte SSD. When I installed an SSD drive in my old MacPro5,1, SSDs were very expensive and I opted for a 500GB instead, and that was my biggest regret. With my 2018 Mac mini, the 2-terabyte option was too expensive, and, again and I opted for one terabyte. Again, my biggest regret. Now I’m running with both my music and audiobook libraries located on a second SSD, but even if Apple allows this configuration, it does not work exactly right, and that is why I never use Apple’s Music and Books apps. When the new silicon Mac mini comes out, I swear, I will get it with the 4-terabyte drive option!
macOS requires a lot of free space for caching, snapshots, etc., which are invisible to users and not reported in the available space.
In the hope that your 500 GB drive is good enough, if you are not doing so right now, keep the Mac running 24/7 so it will have a chance to run maintenance and housekeeping tasks overnight, which clears a lot of disc space. Sleep and power nap will conserve energy and hardware strain. The problem, of course, will not become obvious if you had a larger amount of free disc space. Ergo, it is not a widespread issue with most users.
If the idea is absolutely intolerable for you, then download Onyx (freeware) and run the Maintenance tasks which are basically what macOS does in the wee hours. If this relieves your problem, then your long-term choices are: install a larger startup SSD, or keep your Mac running 24/7, or run Onyx maintenance tasks regularly.
This, of course, is just my guess as to why your Mac is slowing down. As a first step, I suggest doing the Onyx option, and take it from there. If drive space is not the issue, then it’s time to look for other causes. In the short term, if you want to try it, erase the drive once more and reinstall macOS. Before migrating all of your data, try running the Mac barebones. You should still have the default apps that come with the system. If there is no slowdown, chances are, either disc space is the issue, or one or more items in your existing data are the cause.
