Reiserfs was the first journalling FS included in the Linux kernel. Ext3 didn’t make it in until 2001. So reiser was the only game in town for us and why we moved that way. By the time of the murder ext3 was mature so the switch away wasn’t a big deal.
At the time the competition was against a nascent ext3, as I recall, against which reiserfs had significant advantages. Journaling wasn’t standard back then and wasn’t handled by ext2, so there was a lot of competition, and it wasn’t clear that ext3 would be the best solution.
Now that I think about it ext3 wasn’t even a thing. And when it did come on line reiserfs already had a mature journaling system.
I was one of the reiserfs users before it happened and I dumped it quickly. I think an awful lot of people did the same, and since it was one of a number of alternatives to ext I think it is more abandonment than obsolescence.
Now I think my decision was wrong headed, so long as he was no longer involved. The NASA example was given. Alfred Hitchcock movies? Abuser of women but damn good films.
In NixOS you can just mount your directories and run nixos-generate-config.
Try running the same distro on a live USB drive. If it runs faster on USB then it’s likely your HD that’s the problem. As many have said, XFCE is very light weight, if you can’t get KDE running smoothly. As an experiment you could try installing Fedora. The way the installer works is that it boots a live version, which you can use. In the live version you have an icon to install to your system. Use the live version a while, then install and run the installed version. Normally the installed version should be noticably faster.