Connection to CF via German Telekom is very slow

Unfortunately, yes.

All ISP’s that are a direct subsidiary of AS3320 Deutsche Telekom will be seeing these issues.

In addition, networks that are a customer of AS3320 Deutsche Telekom, but do not have another, more direct route to AS13335 Cloudflare, would similarly be seeing these issues…

If you’re paying anything to AS3320 Deutsche Telekom, I suggest you to complain to them, for providing such bad quality, and if you cannot get them to man up, as quoted above, then find another ISP that isn’t depending on, or related to AS3320 Deutsche Telekom in any way.

When/if AS3320 Deutsche Telekom does not want to ensure proper capacity on their network links, or otherwise do not want to peer (locally) with other networks, then there is absolutely nothing that these other networks (including Cloudflare) can do about it.

Got a link to the specific post you’re referring to?