Update 8.22.16 - As of late last week, mob XP has been hotfixed & players can expect generous amounts of experience for boss mobs, as well as decent amount of XP on non-boss shared kills.
We've had the official word, even as Blizzard is quite busy this week, but I didn't need to wait for it either because I've seen the proof with my own eyes: experience gained from invasions has been...savagely nerfed (I think I'm finally done with Warlords now). I've all but written the eulogy for my battle-ready alt army. Yesterday, with the help of a full heirloom set and the +20% XP potion from the garrison, I was able to take my 92ish shaman to 94 by completing three invasions.
Over lunch today I confirmed rumors I'd seen earlier on Twitter: we're getting a fraction of what we saw yesterday. However, there is now a third invasion present on the map, and the timers appear to be resetting every two hours instead of every four. Below is the very rough math I cobbled together:
@cole_pier 4hrs give 320k per w/ 2 opps totaling 640k. 2hrs give 192k per w/ 6 opps (in a 4hr period) totaling 1,152,000k. 2/3— Ross (@elepheagle) August 16, 2016
If we assume that each invasion takes 20 minutes to complete and use the numbers above, and if my math doesn't suck, it shows that yesterday we could expect roughly 16k XP/min during invasions. Today, that number is 9.6k XP/min.
I can understand why: if XP remained the same as it was yesterday, a level 90 player could feasibly see two million XP in that four-hour window; that could mean four to five levels, meaning reaching max level is easily obtainable in an afternoon. Just seems too easy and likely not in line with Blizzard's original intentions.
TL;DR: there's a lot more experience on the table to be had, but we'll have to settle for grabbing it in smaller handfuls, as the overall rate of experience given by invasions has been reduced. At least with the two-hour reset timers there won't be as much downtime for players looking to farm and level. I'm curious to see if this trend will continue next week.