Experience, gonna keep posting this

without boosts I would assume its the same, 5 player vs coop vs ai end at 8 min, tt end at 8 min, or just playing bots. Its the fastest since you win 100% of the time but is quite boring. Otherwise normal blind games are the fastest if you can win a majority of the games. Ive seen people take 3 months though and just spam aram games but that was b4 the xp change.

1- Play the AI games until they expire the AI boost. I suggest playing AI all the way to 30 because of the easy wins. Winning gives more EXP.