font preview extremely slow since CC2018, also for old CC2017 version

You should try de-activating all your fonts and then re-load. Are you using a font manager?

I tried every version back to 2015 now, and it's the same in every version. I assume that some font-engine thing system-wide changed

First, I would recommend trashing your font cache, as a corruption could cascade from cache to the other and degrade performance.  You can find the steps here: https://helpx.adobe.com/photoshop/kb/troubleshoot-fonts-photoshop.html

Next, the reason for font management is because each font takes up a small bit of memory.  A hundred fonts will be less demanding on system resources than 200 or 300.  Thus, unless you have a machine with amazing resources, it's a good idea to deactivate unneeded fonts.

Based on what you're describing -- that things were working well and then suddenly went bad -- I'm inclined to believe the cause is either a corruption in one of the caches (there are two -- one for the OS and one for the app) OR it could be the result of an ill-formed font.  Downloaded fonts are notoriously buggy, particularly those which were created for other writing systems (there's nothing at all wrong with the idea, but a lot of font designers cut corners, particularly for complex scripts like Arabic or Hindi as well as novel scripts like Tolkein Elvish or Klingon).  If you didn't get the font from a reputable font foundry, I would be careful with it.

Hope that helps,
David

I have that issue on two different Computers, and I guess it's very unlikely that on both the cache went bad ;) On my Computer at home, which is way faster, it's not that heavy, but still there. On my other computer I now have to preview a font in an external app, and then chose it directly in PS. (btw, there's 200 fonts installed on that system, same at home, and there wasn't a problem with 300 I had before)