Anti-Virus - Performance Testing April 2013
Microsoft Security Essentials is basic and impacts performance less than many products. Recent product testing rates other AV product performances against the MSE performance base line
QUOTE: However, you use your computer every day, and the last thing you want is protection that slows down everyday tasks. AV-Comparatives researchers have once again put a collection of popular products to the test, identifying which will let you sail along unhindered and which will put a drag on performance.
The report doesn't specifically include Microsoft Security Essentials among the products tested. Rather, the researchers took the case of a Windows 7 installation with MSE active as a baseline for comparison. They found that about a third of the products tested impacted performance less than MSE alone, so replacing the default antivirus with one of these would actually speed up your computer!
Antivirus protection needs to get working as early as possible in the boot process, preferably before any malware processes start. On the other hand, engaging full antivirus protection can slow the boot process. Some products resort to putting off full protection in order to minimize impact on boot time. According to the report, some load their services "very late (even minutes later)," so boot-time testing isn't necessarily relevant.
The report doesn't include boot-time testing, but AV-Comparatives researchers did perform a spot check to see which products actually load their protection as early as possible. They found that all except AVG, Bitdefender, eScan, Kingsoft, Microsoft, and Sophos delayed full protection to some degree. The others permitted the test malware to launch, and whacked it later on after completing their own initialization. I definitely favor completely preventing malware attack to allowing the attack and then trying to undo the damage.