Yes, it does make it slow, and reduces user customizability. I suggested a way that can make it fast and increases user customizability, as well as improving security, and improves backward compatibility, too. Yet, it would seem, they don't like that. (Although, I don't know if the authors of Pale Moon will consider my idea; probably not, but it may be worth a try.)

Have you got any stats to back that up? I've been playing around with it and there is nothing from the profiler in Chrome or Firefox that suggests it might be slow.