Yes, like VOS said the guide data I am talking about is all the data the DVRs have about the shows. This includes all the data for the shows coming on in the next two weeks. All of the shows you have recorded on your DVR currently, all the shows you have recorded and deleted in the last 30 days, and I believe some guide data on the VOD programming available to you if your box is hooked up to the internet.
Your DVR is constantly going over all that guide data searching for upcoming shows it is supposed to record and comparing it to shows you have already recorded and deleted within the last 30 days (it won't record the same episode of a show twice within 30 days even with repeats turned on).
The bigger the hard drive you have in the DVR the more shows/movies you can record. The more you record the more guide data the DVR has to sift through.
As far as speeding it up it seems like you hadn't heard about clearing the NVRAM like ncted posted. While not in DirecTVs manuals it is pretty well known in the forums and you should have been able to find that info here and on DBSTalk pretty easily.
But since you hadn't here are a couple more tricks you might want to try if you haven't.
In your guide options turn scrolling effects off. This tends to speed up the guide and DVR playlist.
In your HDTV settings turn native off and deselect all resolutions except 720p or 1080i (depending on what kind of TV you have). This will usually speed up channel changes a decent amount, but you may notice a slight degredation in picture quality on some channels if you TV has a better upscaler than the DVR.