This morning, when I needed to look something up on my mac in a hurry, my computer was so slow it was almost completely unusable. I then notice that the Sophos icon on the top bar of the mac indicated that Sophos was downloading. I'm assuming it was downloading new virus definitions, but I don't know that for sure.
I have noticed this situation a few times in the past as well. When Sophos is downloading, the mac becomes very very slow.
This is clearly a bug in Sophos - either by design or not. Sophos should be changed to throttle its download activity, so as to not affect the rest of the computer.