No. As long as the pre-show moves people through at the same rate the ride does, it will not slow things down. Think of it this way (all numbers are 100% fake made up):
Let's say the ride loads 10 people per minute and the show holds 10 people and takes 1 minute, there are 100 people in line
Without Pre-show:
100 people/10 people per minute = 10 minutes to serve that line.
With Pre-show:
Total time =
1 preshow (the first group has to see the show before they can board, but successive groups watch the show while others board the ride.
+ total number of people/Min(show rate, ride rate)
= Min(10 per minute, 10 per minute) = 11 minutes...but this minute is because the first group had to see the pre show before the ride could start serving people, over the course of the day this adds practically nothing since people aren't arriving at a rate fast enough to start queuing for the pre-show until a little later.
Essentially, as long as people are going through the preshow faster than people are going through the ride, no it shouldn't make a (significant, since if a pre-show just started that wasn't 100% utilized, and you have to wait for the next one, the average wait increases slightly) difference unless there are no lines at all, then it would take longer til you board, but no big deal there in my opinion. Also, if the preshow processes people much faster than the ride, this insignificant difference becomes even more insignificant.
Sorry for mixing in network type words into this, but that's the application i've studied queue theory with.