-I had no idea that desecrating multiple altars gave diminishing return
This was done for Naga City and vicious Gaan-Telet, where you will encounter every single altar. It's a little bit overboard if you can desecrate all of them.
- I didn't time it, but it sure feels like it took around an hour... are all vicious dungeons like this? o.o
Depends on how quickly you go. It certainly does take considerably longer than other dungeons.
Heh, have you checked out the Vicious Gaan-Telet yet? There is one altar per dungeon floor (excluding the first and last), and you will encounter every altar.