In the first chase scene where Marty is getting away from the Libyan nationalists, it lasts about two minutes before he goes back in time. But when he comes back at the end of the movie and sees himself get into the car and drive away, the chase only lasts about 20 or 30 seconds.
Ok, this is a slipup because the chase is a lot slower, but I mean c'mon, would you really want to watch the whole chase again? (it was about 10 minutes long, not 2 so.....)
Terrorists, Nationalists, Who cares? This was not a slip up, You get the point of the chase. We don't really need to watch another 10 minutes of it to get the idea that Marty got back to see it.
The original chase was dramatized, by showing more of it, different views. Think how much tension you feel when a character is about to get shot, but the cameras show it from several different angles, after all that time, the shooting happens.
Not a slipup
The chase went faster the second time because it wasn't the same Marty (See "How Many Marty's"). This was the Marty who owned the hot Toyota truck. He knew how to wring out a car and get it up to 88 mph fast!
You timed it? Just for the sole purpose of writing something on here? Wow you must have a lot of free time. Why don't you take the same watch and time just how much free time you have?!