It's not just about lack of pace, it's the fact that Grealish and Mahrez are seemingly devoid of playing with any urgency. They hold onto the ball for too long and slow things down.
If you want to create chances at the highest level you have to move the ball quickly and with purpose. At the moment both of them put their on the ball, take two or three touches before playing a square pass. At this point the opposition defence has reset.
Neither of them are also willing to run behind the fullback, which in turn makes us totally one dimensional.