If you are outputting to a regular 1080p monitor, set your playback resolution to 1/4. It will still look beautiful and should play normally. This is how I edited an R3D 4K commercial without any problems.
You should mention what hardware (cpu, memory, drives, graphics card, etc.) you are using. RED media is very demanding and a system that works well for HD may not work well at all for the bigger formats.
On a system that does not have enough ooomph to playback RED smoothly, even when the lowest playback resolution (1/8th) doesn't help, from the CS6 menu do a Sequence / Render Entire Work Area. This will take a lot longer than the quicker "enter key" render, but may provide much smoother playback on a system that cannot deal with non-rendered media in real-time.
On a technical note, RED playback without a full render really taxes the CPU(s) of your system - check out Task Manager while playing back. Once you have done a full render, you only need enough drive speed to server the larger, rendered files for playback.