Pathfinder Society has been doing this forever - one XP per adventure, and 3 XP per level. Leveling feels very fast, possibly too fast (as it does in 5E at low levels).
Exactly. The impression I get is that AL is moving away from a traditional XP system to a PFS-style XP system, and that the final system we get will look very similar to the PFS system with the serial numbers filed off.
The biggest change will be that groups will no longer receive differing amounts of XP for completing the same adventure. I've definitely seen adventures being replayed by players (with new characters) spend significantly more effort in 'metagaming' the adventure to achieve a maximum reward than actually interacting in-character with the adventure, so this could be a net benefit. At the same time, as long as the adventure doesn't end in a TPK, every group will receive the same base reward (treasure may still depend on in-game actions), so for those who feel AL play is meant to develop and reward 'player skill', the new system will feel much less rewarding.
With that said, the biggest disparity in XP awarded for AL adventures currently isn't 'player skill' or the lack thereof, but the number of players at the table versus the table's average level. Seven-player tables generally have a very difficult time earning the max XP for an adventure unless the DM simply declares max XP by fiat (which is something I suspect more and more convention DMs have been doing as time goes by) -- the encounter adjustments in AL adventures are at least in part intended to keep table XP on-target regardless of party size, but in practice, encounter adjustments often end up simply adjusting the difficulty of an encounter without changing the XP much (this is especially true where the encounter adjustment consists solely of changing the HP and/or attack bonus of existing monsters rather than adding or subtracting monsters). Changing the XP system to focus on time rather than difficulty of encounters both allows players to feel equally rewarded for combat or non-combat encounters, and also allows the encounter adjustment system to focus solely on encounter difficulty without having to be concerned with maintaining XP balance across difficulty tiers.
In theory, a time-based XP system could also remove an incentive for optimization and power-gaming, by divorcing a character's XP awards from its ability to deal damage or defeat monsters in combat. Only time will tell if this actually happens, though -- PFS has had an adventure-based XP system for some time, and optimization is still rampant in that organized play structure.
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Pauper
a diabolical plot to get AL members to want to buy their own copy of Xanathar 
