As far as I can tell the direction we're going now is to make the top speeds of the various fighters be reasonably close to each other - energy management included. So the B-wing isn't going to be dreadfully slow compared to the X-wing, for example: the B-wing's base speed is noticeably slower, but you'll get a much bigger boost when you divert all energy to engines than the X-wing would, so most of the time it'll be slower but it can still almost keep up for a while if it really needs to. The X-wing on the other hand has a pretty good regular top speed, but it doesn't gain as much from diverting all energy to engines. A TIE Interceptor or an A-wing would get even smaller speed boosts from doing the same.
Also I think the biggest differences between "fast" and "slow" fighters should be in manouverability, not raw speed. Even if the B-wing was as fast as every other ship, it still wouldn't save you in a dogfight if your manouverability was a lot worse. Besides, speed is the main ingredient in avoiding turret fire, so the B-wing might end up quite useless against bigger ships (which it's supposed to be good at striking) if it was actually a lot slower than the other fighters.
So yes, the B- and Y-wings won't be dreadfully slow. Somewhat slower and less manouverable, sure.