Where would these slowdowns be coming from? The only part I can think of would be landing zones, otherwise you have so much more space to fly in than you do wit

Where would these slowdowns be coming from? The only part I can think of would be landing zones, otherwise you have so much more space to fly in than you do with roads that it's inconceivable that there would be jams, unless you artificially limited flight paths to very narrow "roads" in the sky. And it's trivial to widen those in the case of congestion, in any case.