When the player is levitating, speed effects doesn't do anything.
Steps:
/effect @p levitation 1000
/effect @p speed 1000 100
Result: Normal speed
Linked issues
Comments 5
So it has the same reason why it's faster to sprint than sprint-jump when having speed 2? And not a bug?
Makes sense to me, why would you be able to run/walk faster, while floating.
The same happens while falling, your speed/slowness has no effect while in the air.