Its not simply an fps issue with rift at the moment. It can judder even with 75 fps or more. So far I've been unable to cure it on my machine.
However, your CPU is the bottleneck. Install openhardwaremonitor and check your cpu vs gpu useage. CPU will be maxed out on one thread (shows as 50% load on a core on an i7 with HT enabled) GPU will be 70% or so I expect!