> [is it really] always better to have more speed[?]

I'm reminded of two things that in a way support your premise with the second example being more relevant. Both are outside what’s being discussed exactly, more about knock-on effects maybe, anyway:

The first is that Angry Birds found the speed of launching birds into structures was important for engagement: it cannot be too fast.

The second is from a commenter somewhere who said they had or worked on software that took ten minutes to boot in the morning, which is when employees made coffee and talked and brainstormed. Everyone was disappointed when after refactoring, the software booted immediately.

Wouldn't that just change the shape of the pareto frontier? It stops at some point (when marginal utility becomes negative) - but this is not special, they typically stop at 0 as well.