The conditions they provide are set to cover IMC’s list of issues or test cases. So arguing that with higher rates you could discover more bugs is nonsense, By that logic we’d have to level to the end in every test.
Besides, they had three CBTs with high rates already so it is actually not surprising they chose to test the original system. Not to mention there might even more aspects to it.
I don’t know about the 12 hours claim - some claimed to be over 110 in the chat though - but if you properly make use of crafting, partying, trading, farming, damage mechanics, map data, passive skills, gem and item enhancement you’ll proceed several times faster than on your first try.
An important question here is how many max rank characters they anticipate for the average regular player after a year. Just one, two, four, etc. From what I’ve seen so far I’d say it’s two and as such the addicted ones will have 6 to 9 (obviously, if less then further progress for each char).
All in all there are many things to fix - and if those are taken care of the progression will be even smoother/faster - but the experience rates are quite fine.
Edit: P.S.: There’s still some chance they’ll increase the rates later on.