OK, first things first:
1) Is this a virtual machine, or a "bare metal" install of Fedora 20?
2) When did you first notice the decrease in speed?
3) Are you running critically low on available disk space?
4) Have you made any changes--related either to hardware OR software--over the past few months? Example: running one (or even multiple) virtual machines certainly can slow things down if you don't have enough CPU or RAM.
While there are all sorts of possibilities, I'd start with these... any more information would be very helpful when attempting to diagnose system performance issues.