I thought I had replied to this, but I guess I missed submitting after the preview, What I meant to say in my reply was
I don't see any indication on the speedtest page that it might be impacted by AV. Turning off real time protection in MSE seemed to give one faster test, then it returned to it's normal pattern.
I wonder if some of the wobbliness is down to the network path including both wifi and a noisy and unpredictable homeplug connection. Unfortunately the ADSL router is a little hard to access, so I can't easily run the same test with an ethernet cable patched directly into the router.