In a situation where you are reading from a DVD to write to another device (DVD or HD, say) - there is an additional step where the system will attempt to verify the accuracy of the data written by re-reading from the source.
This extra step is not included (or handled more gracefully) in a playback-only situation. Thus, a small data error on a source may be enough to slow a read-write process down to 0 FPS, yet if it shows up at all on playback, may only affect a frame or two of the sequence, and be missed by just watching playback on a monitor (or TV).
The only way to truly verify a source disk is with a program like Nero Disk Speed 5, which will find small errors like that, even ones that may not generate an error message during the write process (although 0 FPS is almost as good as an error message).
To MonkeyCowboy - when I backed up my copy of Night At The Museum, not a single problem or FPS slowdown - source disk is probably the culprit (check for dirt, finger prints, etc. … the lack of error message points in that direction)