For some reason I'm not having the same experience. Tried different images, only one that doesn't lag in youtube is FriendlyDesktop, but once I run apt upgrade to the latest version, it doesn't handle it anymore.
With friendlycore + mate, I installed chrome and looked chrome://gpu, I don't see hardware accelerated. Do you know what else I need to do?
Sorry I know this doesn't concern armbian, but was kind of wondering if anyone know what else needs to be done to be able to play youtube at least for 1080p.