We haven't had any reported issues so far but it could be the case that nesting RC objects can cause slightly less efficient scenes because the instancing in the nested RC should be disabled. The Nested object is seen as a single combined mesh by the 2nd RC object and if the polycount is very high you may have lost some of the benefit of RC's instancing capabilities.
A couple of other things that may help. Make sure that the nested RC object's default > instancing engine is disabled otherwise the 2nd RC object won't generate instances. Also if it is enabled you won't be able to use the Points-cloud display mode that we use to keep viewport performance responsive. I'd also be sure not to turn on the Nest parameter in the default segment containing the nested RC object unless necessary. This is used to re-seed the randomisation in the nested object for each time it is used, but it disables instancing as well.
If you are able to share the RC objects with us we'll happily take a look at them and try to give you more detailed information about what is slowing down the scene and see if there's anything that can be done to remedy it. If it's a large file please feel free to send it to use via wetransfer, dropbox or another sharing service to firstname.lastname@example.org.
I hope that helps.