For me, when speaker talks slowly, I tend to loose attention. I get bored, start to think about something else or just daydream. Then I have no idea what was told and have to rewind back. Speeding it up helps a lot, I stay engaged.

Imo, speakers in educational videos tend talk very slowly. They do not talk in normal talking speed, they are slowed and speeding it up is basically moving them to normal speed.