Noun to express the frustration of being behind a small group of cars that's driving more slowly than necessary

At your request, I propose:

Slow boat [to China]

On a slow boat to China

On a course or trajectory that will take a very long amount of time, especially with the conclusion or destination being uncertain.

from The Free Dictionary

I've put the "in China" in brackets, suggesting you drop it, because that element pertains to the second (unbolded) part of the definition above, i.e. that the destination is uncertain. This is also why the phrase is sometimes rendered "a slow boat to nowhere".

But in a car trip, your destination is certain. It's the amount of time you want to focus on, not the uncertainty of the destination, which doesn't apply in that scenario.

The phrase actually has an interesting history. We read in the same source:

A very long time. A poker players' expression for a player who constantly lost was “I'd like to get you on a slow boat to China,” meaning that the others would have all the time in the world to win the guy's money.

Composer Frank Loesser used the phrase as the title and the first line of a 1948 romantic ballad, and the expression started being used as a compliment.

The Wikipedia link about about the 1948 song describes a biography of Frank Loesser, its composer, written by his daughter. It pithily captures her conclusion about her father and his buddies’ coinage (or usage) of the phrase:

The idea being that a "slow boat to China" was the longest trip one could imagine.

a drag

A tedious experience, a bore, as in After several thousand times, signing your autograph can be a drag. This seemingly modern term was army slang during the Civil War. The allusion probably is to drag as something that impedes progress. [Colloquial; mid-1800s]