Unfortunately what you described is a common occurence, which plenty of customers of the German Telecom have to suffer from on a daily basis. The interconnections between the Internet provider of the game and German Telecom are congested, which is likely why you experience high latency times. To read more about the topic, please google for "deutsche telekom peering heise" - you can use Google Translate for the articles in the search results.
To test if you are affected by slowdowns and horrid latency, please check the following Blizzard pages:
https://eu.battle.net/support/en/article/7871
https://us-looking-glass.battle.net/
Additionally, you might want to run independent speedtests in order to see how good the interconnections for your Internet connection really are. This is a great speedtest: https://speedtest.seedboxes.cc
You can also use sites like https://speedtest.net Search for "fcd servers" and run that speedtest. The Cogent interconnections are typically congested.