Mobile data highly depends on the carrier. We have three distinct mobile networks: D1 (Telekom), D2 (Vodafone) and E (Telefónica aka O2) and Telekom is the best by far and O2 is a joke although comparably cheap.
We used to have four but about ten years ago (whoa, time flies) Telefónica bought the fourth carrier, e-Plus and the E1 and E2 networks merged. Probably was a smart business move because they’ve become less of a joke since.
As a customer of both Telekom and O2, i highly disagree…the Telekom net most of the times is shittier than the O2 one - even though the O2 one sometimes looses connection all together, in those cases the Telekom connection doesn’t have Internet at all either.