Firefox is slower, not because it's worse, but Gecko is a minority engine in the web (~3-4%) and because of this the most webs are optimized for Blink. That is the only reason and because most current Browsers are using it, a devils circle. The result of leaving Google hands-free for too long and that for 20 years the number of available engines has remained stagnant (3 and some testimonial exotic forks) because it is the most complicated part of a browser. Little can be done now.

Well, Apples WebKit is even worse than Gecko, as a small consolation for FF users.