Accelerator not the same since exhaust repairs

I took my car for repair and had the passenger side exhaust manifold and gasket replaced. However I now notice a significant change in the performance of my gas pedal. Its hard to explain really. I used to be able to "gun it" and my car would shoot off nicely. Now I can not use the same pressure I am used to because it feels like the pedal hits the floor or is hitting something. It also now takes time to reach speed it didn't before.
Before I call the shop to inquire about what they messed up. I need info on what they could have done to alter my performance. What could they have messed with that would ruin my performance?