You’re the closest to the truth. It’s actually a booster propeller for when the plane needs to go super sonic.

For when you’re in a dogfight and your opponent goes to PLAAAID

Instead of 257mph, they lower this one to go 257.5mph. it makes a big difference. I see your confusion on this topic. Sometimes they lower its big brother windmill and then the plane cat reach speeds closer to Mac book 7pro.