There are any number of reasons it might not be performing as well as new which have nothing to do with hardware and will not be addressed by the warranty.
First of all, how full is the hard drive? If it is more than 85% full, that will slow performance. Please tell us how big your hard drive is and how much free space is left.
Secondly, unless you are running El Capitan, open Disk Utility (found in /Applications/Utilities) and run Verify Disk. Let us know if it finds any problems and says your disk needs repairing.
If your disk needs repairing (or if you are running El Capitan) boot while holding down the command-R key combination the select the utilities and first aid and run Repair Disk.