I think that would be anti-aliasing (AA) that was switched OFF in the course of the update. Do you use KDE, GNOME, Fluxbox? In KDE, you can activate anti-aliasing in the System Settings, I guess for GNOME that's also true. Hunt for "Font Settings" and make sure AA is turned on.
If that doesn't work, you can also influence it with a config file in /etc/fonts (in Slackware, yours might be elsewhere). There's a fonts.conf and a local.conf. You can change the latter. Google for the details.
BTW - I prefer what you call "bad" and I hate AA, which is why you had to be more precise what you meant by "terrible".