In most of the turns in a game you dont need the 8/10 min as the time limit gives you, but in the key turns you need it, often you even need more time. So just reducing the time limit isnt an option in my opinion. But there are other ways to get shorter games.

1. Simultaneous turns. Not realistic at the moment, but maybe in the future.

2. Better quick combat. Not realistic at the moment, but maybe in the future.

3. Chess timer that dosent run in combats. By playing fast in the beginning you will have more time in the end. I think this could be a major improvement. Much time is wasted today by people running the clock down or spending 1 to 2 min to double check everything. With this change everybody will end the turn asap, since you never know how much time you will need later. This also means that if you have been fast early you will actually have the time you need in the key turns.

4. Overall timer. A timer that will show how much time each player spent overall. Maybe just revealed after the game. I doubt many players want an image of beeing very slow, so if you can see the actual time spent by each player I think that will motivate many to speed things up.