Cloudflare has a speed test. Try comparing it to other speed tests around the same time. Should reveal if Dtag is congested/throttling your traffic to Cloudflare sites vs others.
Could also check the official Breitbandmessung by the BNetzA
They also have some interesting statistic by provider, etc. on there. https://www.breitbandmessung.de
everfrustrated|1 month ago
https://speed.cloudflare.com/
blazarquasar|1 month ago