In the two weeks the Malwarebytes real time scanners could clash with McAfee's scanner try disabling it in the settings though it gets disabled in a day or two anyway. I use Malwarebytes free as a backup scanner running it manually weekly. Rarely finds anything that McAfee does not but this depends on your surfing habits.
I have both and yes it does slow things down to have both active. So now I only fire up Malwarebytes Pro for the occasional scan.
Let's not forget that LiveSafe provides a much more effective firewall than Windows own also, not to mention other features, such as Anti-Spam, Network Defence, Parental Controls etc.