Runtime Console it's a developer console which helps debugging your game at run-time. It's easy to use and can be customized.
I have tested it on windows 10, android 5.0.1 and WebGL and it works fine.
Despite the fact that it is based on type reflection, it is pretty fast. You can specify which types hold commands so it won't parse all the types in your project.
More information on the forums: https://forum.unity3d.com/threads/released-runtime-console.467031/