Maybe target smaller maps as the part of a whole.
I generally run 8km maps that make up a whole area of over 224km.
Over a billion foliage instances per tile.
Never had the loading take more than a second using world composition and the normal streaming…
They are static and ever present, not generared.
So what are you on about precisely?
Simply having them set to collide or not collide wouldn’t cause loading to change at all unless the loading itself is somehow testing collision to decide if to spawn the item or not.
And if that is the case you have bigger issues somewhere or an engine bug present.