Thanks to windows the complexity of an OS has increased alot. Shadow fading windows, 3D rotating stuff. Compatibility with older OS's. This adds alot of functionality and is never good for the speed. If AmigaOS continued to evolve I'm sure it would be slower, but machines would be faster.
Unless the OS is kept clean and anything you add isn't part of the OS but just a commodity, than it could be small and quick.
