Jun 20 2019
Seems to be better. Solved by the rebooting mentioned by @ema ? I don't experience that slowness anymore.
Jun 19 2019
I honestly do not think that the issue is connected to my computer, my browser or my internet connection, as the slowness is Wikipedia-specific, other sites load fine, and every speed and connectivity test gives good results. The traceroute I posted above also looks fine, I'd say? It's a "gut feeling", but as a longtime Wikipedia user, to me the symptoms really "feel" like they're on the Wikipedia side. For what it's worth, I'm in Switzerland.
The slowness persists today. Still similar experience.
Jun 18 2019
Loading the same page twice in a row: Yes, there are often differences. It's an overall slow experience, but pages often load faster when loading a second time (not always). Everything seems to be faster now, but not blindingly fast. Safemode: I'm now using https://de.wikipedia.org/wiki/Arbeitermuseum for testing and it loads consistently with loading times between 250 and 300 ms, with and without safemode, no difference. I'm not on a Linux or Mac device, but Traceroute is also available under windows ("tracert" command), this is the result:
Some random examples: https://de.wikipedia.org/wiki/Fu%C3%9Fball-Weltmeisterschaft_2022 right now had a loading time of 152158 ms and didn't load completely. The main text of the article is there, but the Wikipedia logo and navigation to the left are missing. The short article https://de.wikipedia.org/wiki/Alkuin_Stillhart did load in 438 ms, which also seems to be pretty slow to me. Some articles did load without the style sheets.
For what it's worth, I can confirm that there are currently often slow loading times in German-language Wikipedia (using Firefox 67.0.3 and previously 67.0.2). But it's intermittent. And not for other websites, but specifically Wikipedia.