It's still not entirely clear whether this means sites the user has not visited previously/frequently, or sites that users do not visit as frequently based on global traffic data. If the latter, it seems like a clear attempt to slow down and worsen the user experience on anything but the big players. I'm surprised that we haven't seen articles on this change mention the obvious problems with pushing users towards centralized sites.
"With this option, Microsoft Edge will apply added security protection to the less visited sites. This feature preserves the user experience for the most popular sites on the web."