Hello
I work mostly with exterior visualization and I use ForestPack for greenery instancing. Almost in every project I have the same problem using IR - if FP's objects (especially grass which normally covers large areas in my project) are not hidden, viewport performance gets dramatically low - even if FP's objects are not renderable and amount of displayed items is pretty low. If they are hidden viewport performance gets normal. I tried to find better solution than hide FP objects every time I want to use IR, and the one I found is switch viewport appearance of the instances to some proxy type from the default Points-Cloud.
I believe solution of this problem should be done by Itoo, but maybe would be good if Corona team will contact them about this issue which connected with Corona's IR.
Hope this information will help to the ones who has this problem, of course if I'm not the only one who has it :)