If you use commands to summon mobs with high follow range (such as this one:

/summon Zombie ~ ~1 ~ {Attributes:[{Name:generic.followRange,Base:160}]}

There is extreme lag when they are attempting to pathfind to players in adventure or survival mode. It appears to get much worse the more mobs are there. The lag is the type when the world becomes unresponsive (i.e. hitting mobs is nearly impossible, blocks keep coming back when you break them, there is delays when you type commands...). There is no fps drop involved with this lag.

Steps to Reproduce:
1. Use the above command to summon a mob. (Or a bunch of them)
2. Go a distance away and switch to survival or adventure, making sure the mobs are visible (so they can see you).
3. Intense lag ensues.

This bug relates to MC-94438, however, it is not the cause of that other bug, because this bug would only occur on custom maps with custom mobs that have increased follow range. The various mob pathfinding changes in the 1.9 snapshots are likely the causes of these bugs.