Recs.
Updated
Godot is a free, open-source, and MIT licensed game development environment for 2D and 3D games.
SpecsUpdate
Pros
Pro Free and open source
Godot is licensed under MIT license. Anyone can grab the source from https://github.com/godotengine/godot, and compile the engine themselves.
Pro Easy to learn scripting language
Godot has their own scripting language called GDScript. The scripting language is easy to learn with Python-like syntax, but it is not Python. It's very powerful, easy to learn, and it's free of unnecessary things because it was custom built for optimized integration with the Godot Engine.
It can be used to add custom behaviors to any object by extending it with scripting, using the built-in editor with syntax highlighting and code completion.
A built-in debugger with breakpoints and stepping can be used and graphs for possible bottlenecks can be checked.
Cons
Con Difficult to optimize
Godot has an OOP architecture. Everything is an object internally and data is spread among many classes, thus it's difficult to optimize (i.e. not cache friendly, difficuly to vectorize or paralellize, etc).
Read about "Data Oriented Design" for more info about the problems and solutions.