Tested this several times outside of Org.
[Kiril, Fury of Beasts], (increases size on buff). kill a mob then mount up on the largest mount you have in an open area and do not move until your buff is over 9-10. Tested as Tauren with Grand Black War Mammoth. Bloodlust likely can increase the lag. Had 10+ people /yelling and /general complaining about lag. Zone seems to lag until you dismount.
as far as I am aware it is the largest in game size modifier, and from what i noticed it is modified by bloodlust/heroism.
spoke to two people that run max graphics at 80+ fps that said they dropped to 10-15 during the testing.