When you re-cast E while it's in the air, it's supposed to immediately detonate when it lands.
Except it doesn't consistently work like this, on a game-to-game basis; sometimes, it detonates half a second after it lands, and other times it detonates immediately. The only thing consistent about it is that it will always behave one way for the entirety of the game.
If you're in a game where it's not detonating immediately, it puts you at a huge disadvantage, because it becomes very hard to land a long range E on anyone. No matter where you throw it, the extra half second gives your target enough time to sidestep it. It's better to manually detonate it when it lands, but it's still not as fast as when it immediately detonates.
If there's some technical reason that this can't be made to work instantly 100% of the time, the mechanic should be removed or normalized to half a second.