Since 1.10, pretty much all servers are lagging really bad. Auction house is broke and when you enter an instance, mobs are not spawned. But they do spawn after a while.
You can run behind a boss before mobs spawn and kill the boss once they spawn. This is pretty useful in places like Dire Maul north for a 5min tribute run, or maraudon for a princess run. I tested it with 45min ud strath, but the timer wont start at the gate. Still useful.
Enjoy!