Totally this. People who can‘t maintain speed drive me nuts. They‘re often the same ones that speed up when you pass them. Them, and for some reason the assholes going randomly 5-10 under the speed limit but suddenly find the gas pedal on the highway merge and make it difficult to get around them.

Not sure what cruise control has to do with this. Different cars have different acceleration speeds and there are conditions that rebalance traffic based on acceleration like traffic light or a slow moving obstacle. Also if you live in high speed highways there's always more space for variability as not everyone is always comfortable pushing the limit.