What version of security centre, Dat and Virusscan are you running? open task manager (ctl alt del) and see if any mcafee program using excessive cpu or memory when these issues occur.
I had just redownloaded everything on Monday because I was one of the folks who lost internet access after that weekend update.
Security Center version 11.0 build 11.0.679 lastupdate: 8/25/2012
Virus scan version 15.0 build 15.0.302 Lastu update 8/29/2012 DAT version 6818 DAT creation date 8/28/2012
Boot DATA version 6802 0000 Boot DAT creation date 8/13/2012
Engine Version 5400.1158
Starting with a quiet system (2% cpu) when I drag the zip file into email, it is mcshield that starts running up the CPU, at 50% (this is a dual thread CPU, so 50% is max a single threaded process can do).
8% was my largest on an i7 775 cpu and 6gb ram. Ok on the beta so will retest later on the same version as yours and a e6600 cpu
In the mean time, is there a way for it to check files that are being created from the browser but not from my backup program, which needs to create humonous tempfiles (dramatically slower with virus check on)?