Levelling in the elemental invasion zones will only be fast if you are in a group and are getting all mob tags.
If you notice mobs going grey and being stolen, just give up and move.
By just questing solo you can potentially do 1-60 easily in a single waking day
I did Warlords of Draenor, it has really dense and fast questing.
For dungeons, you probably want to be running old dungeons
from Classic, The Burning Crusade and Mists of Pandaria.
The more recent dungeons are much harder at the moment even for experienced players
due to really wonky health/damage scaling.