Measure the spring. They relax a bit after you first start using the machine and get a few hours on it. It might need to be adjust. Measure the length of the spring coil from end to end, and only the actual coiled part from where it starts at each end. It should be 9".
Also, don't be fooled into thinking the blades are slowing down in thick grass just because of the sound. If the grass is tall and/or thick, the clippings muffle the sound of the blades buzzing around and make it sound like the deck is slowing down.
If none of that helps, let your dealer take a look at it. It may have a bad belt that is glazed over and not holding the sheaves like it should.