I searched for a 2d android game engine which has an ui designer. Libgdx is cool but designing the game is pretty hard. But which game engine is better? Defold or Citrus? I think Defold uses HTML so I'm worried if the performance is as crappy as in Unity. Is there maybe a better engine with an ui designer and good performance?
No. Defold is a native engine. It has a light footprint and good performance. But like with all engines you will need to be careful about fill-rate and draw calls.
(HTML5 is available as a build target and works by translating the native code to Javascript through Emscripten.)