Remember, there is a ‘Composition Viewer Resolution’ and a ‘RAM Preview Resolution’ and they are independent of each other. Remember the resolution at which the ram preview is generated is changed under the ‘Time Controls’ Panel.
In the likely event that you were already aware of this, its also possible a third party driver/hardware is causing the problem. For instance, the Matrox RTX2 Editing card has WYSIWYG drivers for after affects so that your comp is output to a preview monitor, however when this function is enabled, your comp will never play in realtime, it will play at half speed at best, regardless of how fast your computer is.
Jon Geddes
Motion Graphics Designer
Guava Street Productions, Inc.