Instances are slow in becuase of Material Connections
Instances are an important factor in many workflows. However there seem to be two bottlenecks all deriving from the same issue in Maya regarding instances. One is the Attribute Editor. If you select a transform that has an instanced shape node (that is instanced 1000+ times) Maya stalls for a good 30 seconds if the AE is open. The second issue is if you delete all by history, Maya can stall for 15+ minutes if there is like 15k instances. This is all because each instance can have a different shader assigned to it's instObjGroup array. The delete all by history stalls because of the Delete by Shading Groups and Materials and the AE stalls becuase I am guessing it is iterating over each instance and figuring out which materials to display for that instance in the AE.
So these issues lead me to this suggestion. An option in the shape node that will make it so they all share the same shading info and only 1 instObjGroup. In a lot of productions it's unnecessary to assign different materials to instances and that definately seems to be slowing down Maya.
Wow! I just understood why instances was so slow! It's a shame default shading group have to had real connections on millions of nodes. :(
Sorb [Stefan] commented