On a multi-lane highway, the carpool that cuts in front of you puts one extra car ahead of you, but there was only a 1/N chance that the car they took off the road would've been in your lane otherwise. So yes, they do slow you down.
The other interesting irrationality is that every other car on the road also slows you down by exactly the same amount as the cheater, yet we're rarely as indignant about the other poor saps on the road as we are about carpool cheaters. So there is definitely a fairness aspect to this; we shrug off folks who inconvenience us with minor annoyance as long as they play by "the rules" for inconveniencing us, but we get mad as soon as someone inconveniences us by not playing by the rules.
The difference is that the cheater is only slowing you down because they're cheating. Otherwise they'd be behind you. We understand that limited road access is allocated on a first come first serve basis, and cheaters are subverting that.
The other interesting irrationality is that every other car on the road also slows you down by exactly the same amount as the cheater, yet we're rarely as indignant about the other poor saps on the road as we are about carpool cheaters. So there is definitely a fairness aspect to this; we shrug off folks who inconvenience us with minor annoyance as long as they play by "the rules" for inconveniencing us, but we get mad as soon as someone inconveniences us by not playing by the rules.