Is this bike new to you? It takes a good number of miles and days/weeks/months to get used to riding a recumbent, even if you've been riding bikes.
No inherent reason that an EZ-1 can't keep up with your basic mountain bike, all else being equal.
Is there air in your tires? Do the wheels spin freely? (Hold the front or rear of the bike off the ground and spin the front or rear wheel). Do you ride in a low-enough gear? Or a high-enough gear?
Your brakes aren't rubbing against the rims, are they?