Atomic Game Engine (https://atomicgameengine.com/) also looks interesting since it supports C++ and C# among other languages.
It's almost feature-complete, tons of bugs have been eradicated already, and the stable release is should be 2-3 months away.
As for slow developments... I don't know where you're taking that? Godot has dozens of commits every day, and the pace at which 3.0 features were implemented is honestly impressive (I'm obviously biased as a Godot dev, but check the GitHub repo [0] and the monthly progress reports on the devblog [1]).
Godot has been in development since 2007, open sourced in 2014, and has had 400+ contributors [0] over 3 years vs 32 [2] for Atomic over 2 years. On the other hand, young Atomic is an overlay on the mature Urho3D project with 100+ contributors [3] and over 6 years of development.
Both Godot and Atomic have their strengths and weaknesses, and are IMO two great open source game engines - but as far as maturity goes, I think the sheer size of Godot (both in terms of community and git/issues/PRs activity) compared to Atomic says a lot.
[0] https://github.com/godotengine/godot [1] https://godotengine.org/devblog [2] https://github.com/AtomicGameEngine/AtomicGameEngine [3] https://github.com/urho3d/Urho3D
THUNDERBEAST GAMES began developing the Atomic Game Engine on November 12th, 2014 by forking Urho3D. It was released under the permissive MIT license during GDC 2016. Atomic is now being used in production environments, has 30 contributors, and runs on Windows, macOS, Linux, Android, iOS, and WebGL!