So, I've been testing Godot for a little bit now and I'm starting to really like it. It's super lightweight (i.e. an empty project actually feels "empty", without tons of crap generated around it by default) but still includes many of the important features and lets you easily extend most things with scripting to fit your needs.

As a downside, I'm not a huge fan of dynamic programming languages for game programming. I prefer something which is compiled and more explicit (not everything being a "var").
Their scripting language seems really polished, though and so far I've been getting around with it pretty nicely. Also, from what I know you can actually use c++ or c# for scripting, aswell. It's just not as convinient or easy to use.

In terms of performace I can't really say anything, yet. I haven't run into any issues so far. Then again, all the things that I tried weren't very complex.