Explaining it a fourth time, still kindly, is where most people quit being teachers.

Patience is cheap for the first three attempts and expensive after that, which is why the fourth is the one that counts. A sigh does more damage than the delay ever did, because it teaches the person that asking has a price, and they will stop asking long before they stop being confused. Nobody is slow on purpose.

When the irritation arrives, change the explanation rather than the volume, since a fourth identical attempt suggests your version is the problem. Ask them to describe what they think is happening. That is usually faster than starting again from the top.