Have you made any changes? Wheels/tires? I had a friend who added new wheels/tires on his car.....and they were over 20lbs heavier on the rear than the stock wheels/tires. This unsprung rotational mass mad his car slower.
There are other things, and you could just need a basic tune-up. Filters, plugs, etc. Do you also have a lady on the hood? That could be 100+lbs of added weight slowing it down........