McAfee's gotten a lot better with resource usage in it's recent versions since they finally started pissing off enough customers and realized they needed to do a re-write, but I don't know if Shaw's rebranded version is the latest version or not.
If there's a possibility that advertisements give infections, would browser extensions like AdBlock prevent them?
For known ads, yes, since it stops them from loading.
It's part of the reason a lot of people use those plugins, actually.
