[atlas] probe congestion?

Hi all, while running a DNS UDM on a fixed set of (reused from a previous UDM) probes, I noticed the following. When I start 6 UDMs against the same set using the web UI, as a one-off measurement, starting "now", the RTTs for all of the measurements shoots up on all of the probes (500-1000+ ms). When I start the 6 tests individually, the RTTs are much lower, and close to what I'd expect them to be. It appears to me that when multiple UDMs are scheduled on the same probe, these should be run in serial, not parallel, in order to not run into congestion issues. Or am I simply expecting the wrong behaviour, and should I not schedule one-off measurements in parallel in the first place? ~paul