I could have sworn there was a thread on this already but Im not seeing it so..
I guess I'll start one!
Areas where mobs take a long time to spawn upon entering.
Blizzard Valley, beetles
There's several dragon related quest that send you here to kill the various types of beetles, the spawn is extremely sparse, in an hour or so I only killed maybe 10 or snow beetles often I had to wait several minutes in an area to have one pop. The same is true for Ice beetles a little further on up the road, tho their spawn is a bit more dense once it does appear.
Slate Crest, slate golems
The slate golems in the field near this community are extremely sparse, I flew over there to collect comps for some armor I was making and after the first three I killed I had to wait a few minutes for one to spawn just down the cliff from the first ones then nothing...so I flew to the other side near the machine killed the one over there and then nothing else spawned so back to the first area, killed one...ect all in all I waited about 5 mins in each area for more to spawn but found none.
Tazzon Desert, gem golems
This one Im not sure about, as it could have been just me at the time, but the gem golems seem very sparse within the canyons, but they tend to be plentiful in the handful of spawn spots above it. Like I said I might be wrong about this one and was just landing in areas they dont typically spawn at.