The magician has to wave his hands around

Suppose you hire two agents to find the cheapest hotel for a vacation you're planning. One immediately says this one, $120. The other takes an hour, and comes back and says dang, the first agent must have been lucky, that is indeed the best deal. You're more confident in the second because the effort you've seen more closely matches what you expect an exhaustive search would take.

Absent other information, processing time can one of your best heuristics for the value of that processing.

In real life both of these guys are websites, and it's 3-30 seconds, not an hour. This is apparently an observed pattern: users get results too quickly and are suspicious until developers add a nominal wait and a spinning wheel animation