No, LFS is playing the replay everytime in the same speed. It doesn't matter how many cars are on the track. The replay data will always played in the same speed.
Only the packet size is variable. For example, the data for one car is 160 bytes per second (example too) in the replay. Then in the replay is a time packet with data how much car packets are follow.
So in the one car replay every second there is a TIME-packet and a CAR-packet. In a three car replay: TIME-packet, CAR-packet, CAR-packet, CAR-packet.
When the caster sends every kb the data, it will take 5 secs on the one car replay until the replay stream will be send. On the three car replay the packet size is after 2 secs reached.
So i have the same time data in both replays, only the traffic is higher in the three car replay, because it has more car pakets in for a second.
BUT this is only an example, i'm not sure about the replay data structure. I only know it from the testing, that it's working like this way.
I hope you understand my poor english.