I created a simple blueprint only project to showcase the performance issues I encountered, those who are interested please feel free to play with it:
It’s creating 200 ISM Actors, each containing one ISMC, each in turn contains 200x200=40,000 instances, so altogether a total number of 8 million instances are added to the scene.
The initial spawning process takes a while to complete due to the spawning logic (see PC_ISM event graph for details); when the camera is moved around, the rendering exhibits huge lag in GPU timing, even though no geometry is changed!