Like all things, know your audience.

I know of a case where a delay was introduced because users didn’t believe the task was done because it happened so quickly, and contacted support thinking there was a problem (there wasn’t).

All related support contacts stopped after the artificial delay was introduced.

As you may have guessed, these were not technically savvy users. But it’s what they needed.

There are UX solutions that are not deceptive/time-wasting (e.g. a brief checkmark, but the actual solution will depend on many variables of the exact scenario).

How about dropping CPU clock speed until the computation actually takes avg 5-10 seconds?

This can happen to technically savvy users as well. For instance, when hard disks were only just becoming common, people were used to saving things to floppy disk, which was slow and loud. Hitting save typically meant the whole machine pausing for several seconds while you heard mechanical noises. When using hard drives for the first time, it wasn’t uncommon for people to keep hitting save repeatedly because they suddenly stopped getting the feedback they associated with saving. A similar effect occurs when moving from dial-up to broadband; and from whole page refreshes to Ajax.

In a way, this is kind of an indictment of modern software overall. Why doesn't the average person think that tasks can complete quickly? Because so much of the software they interact with is bloated, slow, terrible crap that they all think that's what is normal.

But on a more constructive note, perhaps instead of introducing artificial delays, you could instead show the user proof that the action has been completed?

Ah yes, I love my "wipe my hard drive and send my wife a text message that divorce papers are on the kitchen counter"-button next to my enter button on my keyboard!

Edit: Humans make mistakes, give them a little time for introspection before something non-reversible happens. Also, for people like you, there's usually an option for "don't show this again".

The feature you are looking for is the "undo" option. Gmail has it, many other apps have it, so you have time to cancel your divorce email.

Pretending to be busy for five seconds with no way to undo it or cancel the request while waiting won't save you your marriage.

There's a difference between "are you sure?" and the time.sleep(10) with a loading bar to make it seem like you're doing work that was done in the first 10 milliseconds.

During testing of a pension calculator I built, the users expressed more trust in the page when it took about 5-10 seconds to show results than when it showed them instantly. They equated instant results with a less complex model.

I've always considered this a good example of the rare case where metrics are true and meaningful (how the users feel), but should still be ignored in favor of principles (well, not ignore - one should look into other avenues to improve the user's perception of the tool). A more black-and-white example would be dark patterns that demonstrably increase user retention.

The unsend-within-a-minute feature in various email services/client is an example of this that is super useful.

That's a grace period - the parent is referring to faux loading times, e.g. a spinner that blocks the UI despite nothing happening.