They do need more attention per person. Doing anything with a group of eight is just more complicated, and you’re more likely to have a person in the group who slows things down for whatever reason plus people trying to help them which can slow things down more.

They’ll take longer per person to order because they’ll be coordinating appetizers and who’s sharing what, etc.

They’re more likely to be celebrating a special occasion which means people less familiar with the restaurant or even people who don’t eat out much at all.

And the group is big enough that people will be having side conversations while one side of the table talks to the server, which also slows things down.