As for the issue at hand (YT being slow), like a sibling post states, that's because they refuse to peer at DECIX where everybody else is. They even tried to frame the NSA scandals as "IT companies, be responsible, keep user data in German networks and peer with us directly (on our terms, which will be expensive for you - they 'forgot' to mention that in the press releases)".
The best way to make DTAG more customer friendly (both upstream and downstream) is to move somewhere else.
Luckily in cities you have 5+ ISPs to choose from (some of them DTAG resellers, but still an improvement), and even on the country side it's typically at least 3.
ALL traffic from DTAG users first runs to a Level3 datacenter in New York, and from there goes to other users.
While all other German ISPs have a peering node at DECIX
(TAT-14 is a transatlantic cable owned by DTAG and Telia)
It’s common knowledge in the industry by now that the DTAG uses their 40 million users as leverage to force companies to directly peer with them (and pay for peering).