Faster would presumably be an issue, yes. Minimizing expected energy input per unit Fun output would presumably also be an issue.

Of course, all of this presumes that the FAI's definition of Fun doesn't definitionally restrict the experience of Fun to 21st-century humans (either as a species, or as a culture, or as individuals).

Unrelatedly, I'm not sure I agree about regret. I can imagine definitions of Fun such that maximizing Fun requires the capacity for regret, for example.