When you say the usage stats record the exact 10MB, do you mean the little pop-up after 10MB file has downloaded?
What does the graph show, i.e. does it show it arriving at 40Mbps or 20Mbps?
There are issues with Kaspersky that would generate double speeds, but first time seen when Avast AV is being used.
Can you try switching off the LAN/WAN mode, by disabling TCP/UDP traffic. This will then only monitor the network card interfaces, and should mean interactions with AV software are less, and usually less problems.