Which style would you say is faster in terms of development time?
Cartoony ones takes less time.
Most development time is in asset creation. Someone has to model all that stuff. I hate rigging skeletons so yea, weight mapping sucks 
.I’d rather do the mesh parts.
Realism or cartoon style comes from the shaders you use and the render engine.
Pretty sure the WoW render engine is an in-house custom after all these years.
I don’t work for WoW or any gaming company. I make 3D assets for fun as a hobby. Normally for still images and some short and basic animations.
Building cartoony models requires usually less polygons, which takes less rendering time. Same with shaders and texture maps as cartoons are cell shaded or simple bright colors.
A cartoony game should have a short production period.