When you use Preview Selected Drawings in the Xsheet the Preview resolution is not taken into account.
For example, if you set the preview resolution to 1/4 the preview is still shown at full resolution.
This is normal. When you set your Preview Resolution through the View menu in the top menu bar, you are setting the resolution for the Render and Play playback. This playback is pixel-based and includes rendered effects. The Preview Selected Drawings playback, that you can access through the Xsheet view, is vector based and therefore will not render any faster by decreasing the playback resolution.
Mac OS X (all)
Animate Pro (all)