If you have a marginally powered PC, the large toolpath in the 2010 screenset can cause some lag if you have a large g-code file loaded (200,000+ lines of code).
But yes, many programs running along with Mach3 can interfere with it's operation.
On older PC, a clean installation of XP with SP3, and nothing else, should run Mach3 just fine on even 10 year old PC's.
I have my control PC on my home network for transferring files, but have never opened a browser or email program on it, and don't install anything that checks for updates on it's own. Be sure to turn off automatic uodates in Windows, as you don't need any of them.