I'm an old IT guy who's been hooked on this game since it came out. I've had to tweek several machines to eek out performance specifically for Rift. Every game out there utilizes their own method of running inside the PC. It doesn't matter if you get crazy framerates with GTA or can sail through COD. That only proves your PC works. It doesn't mean that your PC works with Rift. That said, let's get started.
As with everything PC-wise there is a minimum requirement to hardware and OS to play the game. Please insure that your machine/OS version meets the minimum requirement.
After one has achieved minimum machine requirements, get updates to every driver known, up to and including chipset drivers from Intel or AMD. Also, Rift is a DX10 game, so make sure you have an updated version of DX10 from Microsoft's website.
Network: Rift tries to connect on port 6520. If it can't use 6520 it drops down and tries port 80. Setting up port forwarding on your router isolates your machine and the Rift server from the rest of the network noise. See your router manual about port forwarding and DMZ servers for more details. Firewalls also need to allow this port. There are many excellent firewall threads on the Internet ...this isn't one of them.
Clogging Windows: Windows can be dragged down to a screeching halt by temporary files and registry orphans. Registry cleaner tools and file utilities are in abundance out there. Find one that works for you and eliminate bloat. My favorite right now is one released by YamicSoft (http://yamicsoft.com).
Flash: Update Flash players. Rift uses it. If you have a website using Flash running in the background while playing Rift, expect poor performance if any.
Pagefile: Unless you're running 64G of RAM you'll need a pagefile. In my experience, the best size for a pagefile is 2-1/2 times the size of RAM. It needs to be on your boot drive. For performance, pagefiles should be created only on physical drivers and not on seperate partitions of the same drive. So If you have two harddrives, each one should get a pagefile 1-1/4 the size of RAM. Pagefiles on RAID arrays come with a performance hit.
Background tasks: So you're running ACT, and Vent, and AOL messenger, and Weatherbug desktop, Google search, waiting for a printer, SETI, and trying to mark up your Excel spreadsheet? Call it ADD or multitasking, but do you expect your Chevy Cavalier to pull a trailer uphill in a snowstorm? Get a cheap laptop or use your iPhone for all the accesories and leave the heavywork for the machine.
Other Installs: I've been an engineer in the field of computers since 1979. Because of my addiction to RPGs, I try to have only one game installed on my machine. If there was ever more than one, I never got work done. Almost every game install I've seen installs utilities to download, communicate, talk to Steam, or force game specific settings for that game. And game creators are in competition with each other. They don't care if their install takes down another game. As with anything computer related, the closer you get to a naked installation of just Windows and your game, the closer you'll get to the real performance of your machine. Remember just uninstalling a game will sometimes leave droppings in Windows that requires a registry cleaner to properly excorsize that game from Windows.