Running tcpdump in a VM improves performance of scp transfers

Issue

• When tcpdump is running in a KVM VM and then a transfer with scp is performed the performance is double that found when tcpdump is not running.
• Does not matter if tcpdump is run with promiscuous mode on or off (-p).
• Turning on promiscuous mode but not running tcpdump does not increase performance.
• Only affects scp. Transfers with dd and netcat are always just as fast as the performance found when running scp with tcpdump.