McAfee performs differently on each device, while McAfee may appear to be light on your PC, it may not be the case on someone elses. Also if you're using Windows 7 or 8, you have an advantage over someone with using the old Windows XP OS.
I have often said McAfee places their best interest for Business/Corporate software, and their Consumer software is just a side-sale. While that may not be the case anymore. A lot don't use McAfee because of the bad reviews, or bad experience -only from what I know.
Many bash Microsoft Security Essentials for being basic with lack of zero-day protection and low detection rate. But it does work well enough to protect one's PC, if the user knowingly understand some basic computer security.
This can very well apply to McAfee, and many other security software.
Also some users don't like the number of processes/background tasks running, and often it's compared with other AV's with 3 or less running tasks. So while it may not affect system performance, it's probably a personal preference.
Last thing I should mention, don't fully trust these AV comparison tests, because in the real world, they are extremely unrealistic for the majority of PC users.
Others may have a different view, but I hope this helps.