He is slowed, but the other drivers are slowed more by the need to keep traction in the turn.

nitro_force - friction_force > max_force_before_you_slip_on_that_turn

I bet he was close to passing out on that turn, much like a fighter jet pilot can experience when they pull too many g's in a turn.

Speed through the turn is limited by lateral forces. The wall can provide more lateral force than the tires can. Yes, there's friction, but the cars are fiberglass, it's pretty slippery against a smooth painted wall (and they have a ton of power).

During the course of the race bits of rubber come off the tires and form what is basically a rubbery gravel. The racing line is cleared of it by the passage of the cars. Get down too low or up too high and you start sliding like you would on gravel. So he went high, accepted the slippery-ness and used the wall to keep him going the right direction as he kept the pedal down.

On the racing line, even with a clean track, they still need grip to turn -- he didn't.

Also to note that as it is the end of the race, they aren't on fresh tires and probably most/all of the field didn't have the full grip they would during other portions of the race.

He is slowed down by the friction of the side of his car against the wall. And probably by bits of his car being torn away as it slowly disintegrates. But the engines on these cars are powerful and can easily overcome those effects.

They should add wheels on sides as well to reduce lateral wall frictions!

I'm not sure small wheels would really be an improvement, and large wheels would weigh a lot. So it's entirely possible that a smooth surface is actually the best approach here.

2 Skateboard wheels embedded in the bodywork, to touch the wall front and rear, and off to the races.