Unlimited RT in Final Cut Pro 5.1
The Real-Time Effects (RT) option in the RT pop-up menu of the timeline allows you to adjust the playback quality of real-time effects in Final Cut Pro.
These options allow you to determine which is more important to you, wether it be visual playback quality or the abundance of effects that can be played back in real time.
If you should choose Unrestricted real-time playback, you will maximize the number of effects that you can play back in real time but you also increase the likelihood that your sequence will drop frames during playback. If it is more important to you to view your sequence back at the highest quality, with no drop frames, then you should deselect the Unlimited RT option and select the High playback option.