Keep in mind the differance between ripping & compressing. Ripping will just copy the movie as-is, and will go as fast as the drive can go. Compressing the movie is more CPU dependent than anything; although Nero Recode/DVD Shrinks "fast" modes should (mostly) be able to keep up with the drive if you've got around 3GHz+ (Nero claims 15min to compress on fast mode, on a 3GHz P4, right around what a DVD drive can rip at).
What software (and settings) are you using to compress the movie? And how big was the movie before compressing? Are you just doing a "main-movie" only copy - it'll be faster, and in some cases, with no menu's/special features, the movie can just be copied w/o compressing it.
That could account for the rip time differance's...if the first DVD's main movie only was >4.38GB, and the second DVD's main movie was less..