Congestion manifests as packet queueing as well as packet dropping. 120 ms would be a lot of queuing, especially if we assume the 1000 node cluster is servers on high bandwidth networks, but some network elements are happy to buffer that much without dropping packets.
You could also get a jump to 200 ms round trip if a link in the path goes down and a significantly less optimal route is chosen. Again, 120 ms is a large change, but routing policies don't always result in the best real world results; and while the link status propagates, packets may take longer paths or loop.
You could also get a jump to 200 ms round trip if a link in the path goes down and a significantly less optimal route is chosen. Again, 120 ms is a large change, but routing policies don't always result in the best real world results; and while the link status propagates, packets may take longer paths or loop.