When you use new features in PPT 2007 and save the file as a 97-2003 format,
objects using those new features are converted to images during the "save
as" process. This is so the objects look correct (display correctly) in 2003
and earlier versions -- but they are not editable.

Images contribute to large file sizes. I suspect your converted files are
fairly large. Can you look at them in Windows Explorer and see what the file
size is? Additionally, when you re-open those files in 2007, those items are
again editible. That also contributes to the time it takes to open the file.

Conversion itself ("save as 97-2003 format") can take a few minutes. I don't
know that there's a lot you can do about that itself.

One thing, though -- make sure you have local printer drivers installed. I
don't know that it will help, but it wouldn't be hard to test. Just use the
same driver as in the tutorial here and set that as the default printer
driver:

How to install a local printer driver
http://www.pptfaq.com/FAQ00605.htm