Or I suppose I should say "shouldn't", as making games shorter in League means that each game just ends in one side curbstomping the other by 20-25 minutes with no ability to fight back besides the enemies themselves doing stupid shit. And that's how it is right now: either curbstomping or painstakingly long to the point of exhausting its welcome.
So for those wondering why this can't be the case in League for the game to be healthy, the answer's actually simple:
League's not built for it. It is actually impossible for this to be healthy.
All MOBA's following League's build tend to take the same amount of time when they're well balanced: 30-40 minutes average. So in order to achieve the shorter game times, we'll have to look at one of the very few examples in which games commonly end in half the time, and that's HotS.
So here's how HotS plays out every game:
You spawn, you wait 30 seconds to be able to head to lane while everyone prepares.
In the first actual minute, there is a teamfight.
After this, the players spread out to soak lanes for about 3-4 minutes with 1-2 players roaming between each lane. On two-lane maps there's instead a solo and a quad lane and roaming's minimal.
This ends once the first objective point becomes activated in which case the rest of the game is basically League's endgame: teamfights on teamfights on teamfights on brief breaks to clear before teamfighting again. Then the game ends after this occurs enough times.
League can't do this. League can't do this because you work on an item/laning system. It takes 10 minutes to get halfway to a game phase where a HotS game has already gotten to, and that's just how your game is built. Plain and simple. League is not made to be short, and when it is it's a fucking mess. Trying to make shorter times healthy for your game would just transform it into another game altogether.
That's the foundation you have set up. So please try to focus on making those 30-40 minute games as balanced and enjoyable as possible and stop fretting over length.