List of devices per collector and per site

The python script that builds the dynamic groups shouldn't have any impact unless you're running it on the collector.  It makes a few GETs to find out what is present already and makes 1 call to create the parent group (if it doesn't already exist), 1 call per collector group to create the collector group groups, and 1 call per collector to create the collector group.  So, if you have 30 collectors across 3 groups, it makes up to 34 API calls.

The DS makes one API call for discovery and one for collection. 

All told, they should both be pretty lightweight.