Takedowns on enemies grant permanent Attack Speed.
Offense
+9% Attack Speed
Flex
+5 AD or 9 AP
Defense
+6 Armor
WhyPhase Rush
Phase Rush on Xin Zhao allows you to engage a gank with your [E] and easily stick to your target for repeated damage and [Q] knock-up.
WhyNullifying Orb
Nullifying Orb allows you to make aggressive early ganks mid and bottom as well as take jungle skirmishes against AP junglers like Elise/Sejuani/Zac.
WhyCelerity
Celerity's bonus Movement Speed synergizes well with Phase Rush and Waterwalk giving you massive mobility and ability to stick to gank targets.
WhyWaterwalking
Waterwalking allows you traverse the jungle incredibly quickly and skirmish for neutral objectives with a lot more power and mobility.
WhyTriumph
Triumph's massive mid-fight heal is fantastic for allowing you to jump and kill a backline target while still being able to stay in the fight without an escape.
WhyLegend: Alacrity
The extra bit of Attack Speed on Xin Zhao means a faster jungle clear and burst damage in fights for a quicker [Q] knock up proc.