How are 8man dungeons useless now? OP, did you fail out of middle school math?

AK averages 30-40 minutes depending on group, and gives 100/40 tomes.
CM averages 20-30 minutes depending on group, and gives 100/0 tomes.
Prae averages 18-30 minutes depending on group, and gives 100/0 tomes.
Garuda averages 5-7 minutes depending on group, and gives 15/10 tomes.

It's pretty obvious that once your Myth tomes are capped, CM/Prae are still optimal.

And if your group is great (1 tank, 1 healer, 6 dps), Garuda farming is better than AK.

So again... OP, did you flunk out of middle school math?