The actual pit stop is around 3 seconds. Pit entry and exit being limited to 80 kph (or even to 60 on some circuits) is what costs around 20 seconds (again, this varies by circuit).

The stop itself is only 2-3 seconds, but the total time loss is about 20 seconds because there is a speed limit in the pit lane.

Also, doing more pit stops means that you exit the pit lane behind the traffic, wnd you lose more time when overtaking.

You also have to account for the time going in and out the pitlane. All in all, you lose 20s roughly. Those seconds you would have to make up for with the new tires.

Usually you can ignore Pirellis predictions, they are not worth much. It’s a theoretical model, in practice you have to account for weather changes, safety cars, the gap you will return to after the pit stop, defending from or executing under cuts and several more factors.