I did a little research and if you have FPS in Rift that is lower than other games or if you have spikes in your FPS where it jumps up and dips sharply at times, a few changes in your config file can help.
The file is in your Rift game directory and is called Rift.cfg. You can open it with notepad and you should make a copy of it before changing anything. In the file, you are looking to change the following variables to these values:
VfxCullDistance = 256
VFXLimit = 75000
LightingComplexity = 10.000000
MainThreadCPU = 0
VfxCullDistance might not exist. Just add the line above VFXLimit.
What this does is take advantage of the high memory in most graphics cards to load effects and keep them loaded. With smaller values, the effects load/unload repeatedly at the edge their range and this causes dramatic drops in FPS as your graphics card works to load things at the proper distance. MainThreadCPU = 0 lets the game choose the processor core that is under the least load. Thanks to Xensimilla for that one.
Thus, by increasing these values, you should see an increase in performance and not have the wild swings in FPS. It's worked well for me. Hope it helps.