I don't know if anyone else has come across this, but it looks to me as if the most recent driver that RME has issued (3.069) does not work very well with SONAR.
The symptom is very slow loading of a project, followed by very, very sluggish performance. Clicking on anything results in the spinning wheel for at least 30 seconds or so. Then, once I finally get playback to start, there's no sound.
Once this happens, the FF800 doesn't work properly with any other application: Sound Forge, Windows Media Player, or anything using audio. All of these seem to work fine until SONAR is started, or after rebooting.
The infuriating thing is that it doesn't happen each and every time. I can work for hours without problems, then everything stops.
I went through everything, though probably not in the optimum order. Reinstalling SONAR, deleting AUD.INI, whatever. Once it occurred to me to roll back the Fireface driver, everything was OK.
I hope this helps someone.