I worked on some software that provided results to some calculations to general web users, not experts. The calcs were done in miliseconds.

We had to introduce an artificial delay of ~30 seconds to make it seem like it was taking a while to calculate, because users were complaining that it was too fast. They either didn't believe we really did the calcs, or they thought the system must have broken so they didn't trust the results.

This is one reason UIs have animations added, the kind that technical users like to complain about or remove. By making things feel more physically grounded they prevent users from getting lost and confused and give them more intuition about things.

In your case you could show more intermediate values, graph things, etc.