For those using the open-source (Intel / Radeon / Nouveau) graphics drivers, there should be a performance boost in moving to Fedora 20 -- unless you're already running the very latest upgraded packages on Mesa. Going from Linux 3.9 to Linux 3.11 (or 3.12 with F20 updates) there has been many improvements to the open-source DRM graphics drivers. The Mesa changes aren't as significant since sadly Fedora 20 is still shipping with a Mesa 9.2 point release. It's a pity there isn't Mesa 10.0 in Fedora 20 as the numbers would be even higher.
The performance improvements are especially there if you're using newer graphics hardware -- like Intel Haswell or the AMD RadeonSI HD 7000 series hardware.
Even the Intel Core i5 3470 "Ivy Bridge" HD Graphics 2500 performance saw very measurable performance improvements out of the Fedora 20 upgrade.