MR stand-ins through ICE always been heavy, since day one. If this is still the same, MR stand-in is invoked by 'ShapeInstanceTime' attribute inside compound (all compounds under geometry instancing), generally it's used for getting an animation offset in ICE, by creating a lot of standins for each expected frame, or something like this....
Instanced instances in ICE also always been heavy (how you got them, btw...).
Solution for first, it was to simply do not invoke stand-in, by using a plain 'instance shape', probably by supplying some criteria for ID of this node, if there is more than one member in instance group (let's say, get group> get kine global > get array size > modulo "self.ID" by this value).
All that is explained many times on public forums, in all these years.
make no difference, the opposite, if you use only instance shape, the ram is filled faster and rendering tooks way longer.