I suspect that DragonRuby is faster as a 2D game engine and doesn't make sense for a complex 3D game.
I don't believe that either. Unity works just fine for Cuphead, which is a pretty graphically intense game. Plenty of other graphical complicated 2d games too.
I think you're moving the goalposts. Of course Unity can work "just fine" for things like Cuphead. However, DragonRuby is an mRuby wrapper around SDL by the guy who wrote SDL vs. a 3D engine that was reworked into a 2D engine?
Surely Unity has more overhead and DragonRuby can make more optimizations.
On some tiny synthetic benchmark maybe.
Come back when your engine can handle something even 1% as complex, as say, Dyson Sphere Program.