Bitttymacod
[...]I'd wonder if I needed to drill through the Avast firewall, but that wouldn't explain the fact that it happened while I was still using McAffee and that the change happened while using McAffee.
I will continue thinking on this matter.
3 more thoughts from me:
A major issue i had with another firewall was that it did DNS lookups for every connection. That particular firewall tried DNS lookups on every UDP packet (Ryzom uses a lot of UDP communication which doesn't use connections the same way TCP does). They stacked up until response times went ugly. You could try disabling DNS lookup for the firewall. They are for readability purposes (log, messages) only and not required for security.
Another thought: Maybe Microsoft Update auto-installed some windows defender stuff? That one should be disabled if there is another antivirus solution in place.
Third: You said your antivirus takes about 50% cpu (at least i remember you said...?) Find out what exactly does it do using that much cpu time.