I tried but couldn't find any video on raptor start back then when writing this guide, googling it now gives me a Karasmai video titled "Why you should not start raptor on Kayn", checking the KaynMain subreddit yields the same opinion.
Tbh Ionian is the only choice for Blue Kayn lol, the cdr powerspike it gives him is too valuable.

1) A typical Kayn's build with 2 items[*] will yield you about 42~57 Haste (Gore + Death's Dance/Black Cleaver), having the 20 ability haste from the boot will equal to a 9% skill reduction at mid game.
Thanks to the ability haste buff from gore at late game your build will have at least 90 Ability haste in the worst case scenario (105 typical), which means the raw stat from Ionian boot has a diminishing return value (5% skill reduction), to me is not as valuable as tenacity in the long run.
2) Kayn's two big weakness are getting kitted and getting focused on before he could deal damage/using Gore to life steal, tenacity helps with that.
3) The most common secondary rune on Kayn is Cosmic Insight since it reduces smite's cooldown from 90 to 76 second, that can help you getting 900 dmg smite before their jungler, buying lucid boot furthermore reduce it by a mere 7 seconds, but at that point you already have the fifth smite so the summoner spell's haste is not that much valuable.
Tenacity is hard to get in game, while any build will gives you a **** tons of ability haste, so I actually had a bias toward tenacity -w-.
I will update the guide to include the scenario when enemy team does not have hard cc.