Ted Spencer
Veteran
Do you have the timeline set to Unlimited RT? That could be your issue.
I've tried both R/T choices repeatedly, and virtually all sub-choice/combinations likewise (dynamic, etc.). No dice.
Sounds like both drives you're describing are quite fast, but I don't think that's the issue. The timeline goes red ('needs render') the moment I add the very first fade keyframe, even if it doesn't actually fade anything. With playback stopped. Most frustrating.
And again, with identical hardware FCPX runs like champ. I'm going to try the PP6 demo shortly.