Some companies (mine included) have directories where the virus check doesnt scan or at least runs in reduced mode. McAfee in particular doesnt seem to get along with Micro Focus. I run VMs (on my Corporate laptop) with normal Microsoft AntiVIrus and/or Linux - and indeed these do not interfere as much as McAfee.
Having said all this - be sure to run your scan from the CLI like
sourceanalyzer -b pants -verbose -debug -logfile tran.log devenv myProject.sln /REBUILD Debug
then either review the tran_FortifySupport.log or send it to the folks at Micro Focus support.
I would look for things like "minutes to" expand a rule. That shows your system is struggling with RAM.
Also look at any memory exceptions - scanning the code itself (with a virus scanner) should not massively slow things down.
Not having enough resources / not tuning your scan WILL slow you down!