> How does the performance of F3D compare to say rendering the same object in Unreal Engine’s static mesh viewer?

Hard to say since I never used UE static mesh viewer, but if you test it I would apreciate a feedback!

> Could I use F3D as the rendering engine for a game?

Not really. F3D rely on VTK for rendering, which is definitely not a game rendering engine.

That being said, you can easily render 2D sprite for your games by scripting F3D.