Having been in the computer service and repair business for 25 years I can tell you from personal experience that Microsoft Security Essentials is near worthless. I have lost track of the number of computers I have had to clean up from viruses that had MSE installed. And, yes, MSE can slow systems down significantly. It may not slow down your or someone else's computer but I have seen it. A lot.
Avast is good. It's what I am currently putting on my customer's computers. AVG is decent. I have had no bad experiences with NOD. Norton and McAffee are pretty worthless and do slow your computer down.