Push/Pull weapons doesn't seem to allow a ship to go faster than 1x server speed. As a side-effect, using a self pushing cannon (or any other similar weapon) also causes your ship's speed to reset to 1x server speed if it flew faster than that.
It's funny, but I don't think it's intended. Honestly, someone must have noticed it before me but I haven't seen any report, so hopefully this isn't a duplicate.
note: this doesn't seem to happen with stop effects or defensive push/pull effects
simple blueprint to reproduce (fly straight ahead to be sure not to miss yourself with the docked cannon):