You can minimize the screen update time by reducing the number of data items that are polled by RSLogix500. If you close all data tables, and all ladder views, and put just the items you want to see in a CDM, the update time is noticeably faster.
This is most apparent on slower communication drivers like DF1. RSLogix can get really slow if you open up ten or twleve data tables, even if they are "behind" the window you are viewing, the information they contain gets added to the list of items being polled through RSLinx.
And, like Ron described, communication packets must be organized and passed to RSLinx, then polled over the communication channel, then the responses returned, and only then can the data be displayed.
Depending on which RSLinx driver you're using, there may be settings that can enhance the "freshness" of the display as well.
Look around in RSLinx at your driver configuration for more information.
Paul