I recently started experiencing this. I brought it some Megescans content and some of them (but not all) get stuck on this for a long time when trying to open them or drag them in to the level.
I did find a way to make it happen all at once by selecting all the meshes that would have this issue and then right clicking them. It’ll then process all the selected ones and then you probably don’t gotta worry about it again. It’ll take a good long while to get through them though, so don’t do it when you’re expecting to do some work.
Did you ever find a better solution?