Imagine asking a person a math question like what 2 times 3 times 7 is (without you knowing the answer). If that person immediately goes like „42“ you‘ll most likely think that it’s a joke response and the person doesn’t take your question seriously. If however that person takes a few seconds to think you are much more likely to believe the answer.
First reason is just poor UI design. I’m sure there are billion ways to indicate a successful action even if it was immediate.
Imagine asking a person a math question like what 2 times 3 times 7 is (without you knowing the answer). If that person immediately goes like „42“ you‘ll most likely think that it’s a joke response and the person doesn’t take your question seriously. If however that person takes a few seconds to think you are much more likely to believe the answer.
With your overly simple example I would totally believe that person. With harder problems perhaps. Besides, machines are not human.