The problem I see with Riot wanting shorter average game times
League is a game that's late-heavy by design. In terms of gameplay, the champion you pick will be the most different from all other champions at level 18 with 6 items. That's the point where all of your abilities are maxed and your scalings capped.
Conversely, your champion will be the most like all other champions at level 1 with 0 items.
So in the context of reducing the average game time, a by-product is that games feel very similar regardless of the champion you pick because most of the time spent in game is in the early parts where champions are the most similar.
The result is games feeling much more bland and repetitive, coming down more to who drafted the best stats regardless of how the comp operates when the champions really start differentiating themselves past the 20 min mark. No one gets to play their champion except maybe the 1 or 2 players that got extremely fed.
I'm not trying to bash Riot here though. There's nothing inherently wrong with wanting shorter game times, but the fundamental design of the game contradicts short games. Hypothetically if they wanted shorter games without compromising a meaningful draft and strategic champion picks, they could invert the differentiation curve so champions are the most different at level 1 and the most similar at level 18 with 6 items. A change like this would mean that "late-game carries" wouldn't build damage, they would be inherently damaging at all points in the game (maybe by dealing % health damage or something) and be extremely squishy, forcing them to itemize tank and defensive stats so they don't get 1-shot by everything. "Tanks" would be the opposite. They would be inherently tanky regardless of the items they build but would generally favor itemizing damage so they could actually threaten the backline they like to get on top of.
I think Riot just needs to pick one or the other. Either longer games with the current design or shorter games with massive overhauls to how the champion classes scale and itemize.