Apple Final Cut Pro 6 - Manual (Page 1428)

Chapter 28 Using RT Extreme 643
IV
If neither In nor Out point is set in the Timeline:
Final Cut Pro uses the position of the playhead, caching half of the duration specified
by the Range slider before the position of the playhead, and half after the playhead, for
playback in the QuickView tab.
Video is cached to RAM as it’s played. Once the portion of the sequence that you’ve
specified for QuickView playback has played all the way through, subsequent loops
play much faster because they’re being played back directly from RAM.