Imagine a long rubber band attached to 1st and last place. The faster the lead driver goes, the more last place holds him back, but last place (and everyone in between) have the effect of the rubber band pulling them forward and closing the gap.
I think it's turned on from the race's start but obviously it's not as noticeable with the pack being closer. Only when it spreads out a bit.
I did a bit of testing with boost set on weak at the recent high speed ring quick match. If the field were a decent pace, low 3:20s were possible for the whole race but if I (or someone else) parked up on the first lap, 3:30 seemed to be the benchmark time. Even with the quickest cars. If I was in last and then did a lap, I could do under 1 minute laps compared to the regular 1:04s if I running was with the rest of the field.