Thanks for testing! Since 7.16.2 and 7.15.3 give the same results as 7.18.1, it doesn’t seem to be a version-specific issue. But the fact that 6.49.18 performs so much better with the exact same settings is really weird.
It might be worth checking if there have been any changes to the default TCP settings between these versions. Have you tried adjusting TCP window sizes or disabling FastTrack to see if that makes a difference?
Anyway, I’ll take a closer look and see if I can spot anything specific in a couple of customer instances when I get the chance.