MJWIndycars drive on ovals (and this year for the first time on street tracks).They don`t really need to get off the pedal on a oval.These cars are fully made for oval racing (better aerodynamics).This gives them a higher speed.
Formula 1 races don`t have ovals,so the speed is lower.They need to brake,and need more downforce for cornering.This brings down the speed.
What I want to say is that Indycars are only faster on ovals,because they go flat out with less downforce and better airflow than a F1 car.That`s why these F1 cars can`t go that fast.
Thanks.