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Do Virus Scanners Slow Down Your System?

Do Virus Scanners Slow Down Your System?
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Does the presence of a virus scanner guarantee reduced performance, or does it have a negligible impact? We test 10 different products to see if you’re unknowingly suffering with security software.

Remember the days of Windows 98, when CPUs ran at triple-digit MHz speeds and slogged along with less than a gigabyte of RAM? Installing a resident program like a virus scanner often meant committing performance suicide. And heaven forbid a scheduled scan start up while you were actually at your desk. Productivity could literally grind to a halt. At least that’s how I remember things through the fog of time.

Today's personal computers are much more powerful than they were a few years ago, so perhaps the notion that an anti-virus application will still have a debilitating effect on performance is obsolete. Still, folks who began using computers after multi-core CPUs and gigabytes of RAM became the norm have likely never used a PC without a virus scanner installed. They'd have no way to relate to the days of running lean and mean to keep speed manageable. Now we have resources to spare. Cores sit idle, waiting for a task to execute, while low prices on memory make 6 GB and 8 GB kits affordable for even mainstream users.

We should make this perfectly clear: while it’s undeniable that an active virus scan can cause a heavy performance burden, what we’re really curious about is whether or not performance is affected when a system scan is not running. Does it take longer to open files when you have a resident virus scanner installed? Does the presence of the software tax CPU resources while you’re running other programs? What kind of tasks are most affected by security products, if any?

When faced with these sorts of questions, it’s only natural that we’d run some tests to unearth the real answers—this is Tom’s Hardware, after all. So let’s look a little deeper into quantifying the anti-virus conundrum.

What Does A Virus Scanner Do?

Before we begin our tests, we should at least consider how virus scanners work so that we can see if the results are in sync with our expectations.

There are two main mechanisms that most virus scanners use in order to keep your system safe: file checking and behavior monitoring.

File checking is by far the most prevalent technique. The idea is simple: the virus scanner examines the files on your PC for known threats, a threat being a signature of code that is associated with a particular virus. Because new viruses are being released all the time, most virus scanners will periodically download updates containing the new threat signatures.

How could file checking affect performance? Typically, a virus scanner will examine files for threat signatures every time a file is written, opened, closed, or emailed, or when a virus scan occurs. It thus makes sense to predict that applications accessing files on a regular basis might be slowed down by anti-virus software. Conversely, programs that don't involve a lot of file access might then remain relatively unaffected by the presence of a virus scanner.

Behavior monitoring is the second technology that anti-virus software employs to identify threats. This is a pre-emptive strategy to deal with viruses that have not yet been identified or added to the threat-signature dictionary. The virus scanner monitors the system for suspicious behavior, such as the alteration of executable files. This virus-prevention technique probably has very little effect on system performance, since suspicious behavior is probably somewhat rare.

That should be enough of a top-down overview to get us started. Let's get on with the tests!

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Top Comments
  • 38 Hide
    Hupiscratch , November 30, 2010 6:44 AM
    I think Microsoft Security Essentials should be included if possible and there is a situation that I think it is greatly affected by anti-virus software: Windows start-up.
  • 32 Hide
    aznshinobi , November 30, 2010 5:48 AM
    Avast please?
  • 24 Hide
    iam2thecrowe , November 30, 2010 5:31 AM
    before i read the article, my guess is Norton is the slowest and most useless....
Other Comments
  • 5 Hide
    theshonen8899 , November 30, 2010 5:30 AM
    Great article, thanks!
  • 24 Hide
    iam2thecrowe , November 30, 2010 5:31 AM
    before i read the article, my guess is Norton is the slowest and most useless....
  • 24 Hide
    tony singh , November 30, 2010 5:41 AM
    How can u forget Avira , it's so popular & so good .
  • -3 Hide
    alyoshka , November 30, 2010 5:43 AM
    I guess the new ones are lighter than the earlier ones for some of them....
  • 4 Hide
    ruffopurititiwang , November 30, 2010 5:45 AM
    This is the kind of article that keeps me coming back to Tom's! Kudos!
  • 32 Hide
    aznshinobi , November 30, 2010 5:48 AM
    Avast please?
  • 4 Hide
    micr0be , November 30, 2010 5:49 AM
    talk about heavy modifications on the new set of AVs compared to the older ones ... my surprise is norton which i was expecting to cripple the system to a halt .... very nice article btw
  • 2 Hide
    tony singh , November 30, 2010 6:00 AM
    @Fip - Because when dirty viruses do their job, you'll get a headache.
  • 13 Hide
    iam2thecrowe , November 30, 2010 6:05 AM
    iam2thecrowebefore i read the article, my guess is Norton is the slowest and most useless....

    well i am really surprised
  • 2 Hide
    takeapieandrun , November 30, 2010 6:24 AM
    iam2thecrowewell i am really surprised

    I get Norton Security Suite free with Comcast. I was kind of bummed when I found out that's all they have available, but so far its been good to me. I haven't noticed any adverse effects, maybe startup is s little slower.
  • 24 Hide
    apache_lives , November 30, 2010 6:31 AM
    this is tested on a fresh install - the average system has a ~2 year old install and fragmentation and lower end hdd's, combind with a crapload of other software trying to startup - no really a real world benchmark.
  • 9 Hide
    Anonymous , November 30, 2010 6:31 AM
    it would have been useful to see a difference in the benchmarks using different HDDs like the 5400 RPM laptop ones, 7200 RPM and SSDs, that would have made a difference
  • 4 Hide
    Anonymous , November 30, 2010 6:43 AM
    Well the biggest slowdown you will experience with antivirus software is when you open a folder full of exe files and explorer tries to show all the icons of the executables. There is a very noticeable slowdown in that case.
    Also i would have liked a startup benchmark, because the antivirus also slowdowns somewhat the startup process.
  • 38 Hide
    Hupiscratch , November 30, 2010 6:44 AM
    I think Microsoft Security Essentials should be included if possible and there is a situation that I think it is greatly affected by anti-virus software: Windows start-up.
  • 5 Hide
    cjl , November 30, 2010 6:58 AM
    iam2thecrowewell i am really surprised

    Norton has VASTLY improved compared to what it used to be. I use Norton 360, and I have to say that it has been a great product.
  • -7 Hide
    Anonymous , November 30, 2010 7:04 AM
    2 ddragoonss
    thanks,
    AV is far from 100% protection, and could bring new problems (recently ESET NOD Smart Security causes problem with internet connection due to connection inspection / filtering ... ) For IT Pro is risk to get a virus very low, and if get one - few hours to get it out is worh instead of years of boring my pc with AV software
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