When mobs are packed densely (as in many popular farm designs), server CPU usage is absurdly high.
Repro steps:
Start a local default-parameter flat world (I used seed 0 and was in Creative with no structures)
Swap to desktop, use Task Manager to note idle CPU usage (for me this was 40% on one thread of one core)
Make a 1x2x1 volume (I used dirt)
Use cow spawners to place 16 cows in the volume
Swap back to Task Manager, note problematically higher idle CPU usage (This was ~65%)
Drop in another 48 cows
Note much higher CPU usage (I also noted the load bounces somewhat between cores under this load)
On a map that was running fine on Realms before the 1.9 update, I now see 10-15 game ticks per second. Emptying the high-density cow and chicken farms lets it get back up to 20 TPS.
I tried to carefully repro this, and it's not exactly consistent. I used a splash potion to empty a second cow silo and then refilled it, and the CPU usage wasn't as high as before. This definitely repros with a full minute's worth of cow spawners in the silo though on Realms, and 1.8 handled it alright. I also could get tick stoppage with only 16 cows and a very modest amount of Redstone in the original repro Realm.
Please note that I am not saying dense farms shouldn't cost CPU; I'm saying that the cost has been multiplied immensely by v1.9.
Is this still an issue in the most recent versions (currently that is 1.10.2, or 16w42a) of Minecraft? If so, please update the affected versions and help us keeping this ticket updated from time to time. If you are the owner/reporter of this ticket, you can modify the affected version(s) yourself.