While the management side of McAfee may be great the client side scanning engine is a massive resource hog. Having done tests with a particular piece of work the completion rate with McAffee enabled was twice as long. Utterly unacceptable. Even with on-demand scanning turned off the overhead was still bad. Frequent cases of multi-second lock-ups while doing things (which when typing a line of text can be very distracting). Its not like I have a slow machine (2 * Xeon with 4 cores each). Overall the running cost for this AV is just too high as it impacts productivity too much.
Long time since I used Sophos, but it didn't cause any issues like this when I did.
What version were you testing and what OS? I've never seen this and we've been using McAfee for ages now!