As a long-time Blender user, Nim and Godot fan - I'm unsure what you mean. Blender already has a well documented Python API. Godot has Nim bindings, which others here have already linked to.
Yes, Python is slow - but it is a scripting language that can be reloaded dynamically - which makes it ideal as an embedded language inside of an application such as Blender. I'm not sure how Nim would compare in this department, or even how it would get embedded in Blender.
