The person explaining slowly has decided your understanding deserves their time.

Speed in an explanation usually serves the explainer. Going fast covers more ground, demonstrates fluency and gets the interaction finished, and it leaves the listener to keep up or pretend. Someone who slows down has made a different judgment about whose time matters, and that judgment costs them something real. It is a form of respect that never announces itself.

When you next explain something you know well, cut the material in half and double the pauses. And if a person has been patient with you, telling them it helped makes it likelier they will do it again for someone else.