Janna and Nami are the best example how powerfull cast and travel time is in this game.
Both Champions do litterally the same thing, their kit is as similar as it can get without being a copy, Nami even wins hard by numbers alone, she heals instead of shield and her Ult has more range...
And still Janna wins in popularity and winrate.
Basicly all because all of Jannas CC is instant (unless you wish to charge the Q) while Nami has a delay.
No, I am not asking to buff one or nerf the other, I just noticed, there is no better example in the game to show how important cast and/or travel time of spells is.