To clarify, statsite does spawn threads to do some of the aggregation and flushing at the end of the collection interval, but yes it is possible to saturate the main loop at a very high ingest rate.

This. We recently had to upgrade to a c4 instance in AWS because the single core on a c3 couldn't take what we were sending. We needed something like Brubeck because eventually the c4 will run out of headroom too.