[skohan]

I don't think that's true. If anything a tightly-coupled monolith can hurt performance.

For instance have you ever read through the movement code in Unreal? There are a million branching cases for every kind of movement: on particular slopes, swimming, flying etc. If your game doesn't have underwater motion, you're still executing those branches just because other games might need it.

General purpose game engines are by necessity not optimized for performance. They're optimized for being able to support every arbitrary type of game.