This could be completely normal. There's no way to know unless we know something about what you are trying to ram preview. Without the details of your composition and details on the source footage it is impossible to know if this anything other than normal behavior.
20 seconds of footage from a gh2 AVCHD 24p 60mbps with warp stabilization. It has never taken this long to start rendering ram preview on this footage.
Your warp stabilizer may not have completed it task and been cached. That's still not much to go on.
As a good practice, when you use warp stabilizer on a piece of footage it is advisible to render a DI on a suitable production codec.
The computer wasn't using any resources.
A restart fixed the issue. It would be nice to know what the cause was though.
Thanks for your time.