Well obviously if it's a new person, you can't go as fast in the first place so you don't have a problem of it going too fast. Unless you have stupid tanks, then you can't save that. Besides, fun is subjective to the person, you can't just assume going slow is funner for everyone.

Going faster = less entertaining for some people by pushing their limits = more fun; don't just assume everyone who's new would be stressed or discouraged by not matching DPS with their party members.
The best thing is to ask the group, really.