The OE 6.0.1 image is the best-performing release for the AppleTV that I created as it has hacks like nVidia shader caching and a startup sequence in Kodi designed to force things into swap to create free RAM headroom that helps subsequent playback. However it's all a fudge/compromse and both OE and OSMC have the same fundamental issue: the CHD drivers are not perfect and 256MB RAM is not enough for good Kodi operation.
NB: An RPi Zero (€15 new, prob. cheaper used) with 512MB RAM would be a huge improvement over a mk1 AppleTV.