Do Virus Scanners Slow Down Your System?

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years ago, so perhaps the notion that an anti-virus application will still have a debilitating effect on performance is obsolete.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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  • theshonen8899
    Great article, thanks!
  • iam2thecrowe
    before i read the article, my guess is Norton is the slowest and most useless....
  • tony singh
    How can u forget Avira , it's so popular & so good .
  • alyoshka
    I guess the new ones are lighter than the earlier ones for some of them....
  • well from my point of view - antivirus scanner do application loading to take a much longer time and this was proven by your tests.

    I think that AV software has no place into todays operating systems except for inexperincied users. I'm investing money to fast SSD disc to improve performace, why the hell intstall AV software to push performance back?
  • ruffopurititiwang
    This is the kind of article that keeps me coming back to Tom's! Kudos!
  • aznshinobi
    Avast please?
  • micr0be
    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
  • The test rig's CPU looks funny to me.

    Athlon II X4 645
    3.5 GHz, Quad-Core, 6 MB L3 Cache

    Isn't that a Phenom?
  • tony singh
    @Fip - Because when dirty viruses do their job, you'll get a headache.
  • iam2thecrowe
    iam2thecrowebefore i read the article, my guess is Norton is the slowest and most useless....

    well i am really surprised
  • @tony singh
    yes - for inexperienced user. I have no problem with viruses over 8 years. OS Patches, working under different than admin account, not using IE, and not executing every garbage downloaded from internet. And finally virtustotal page for testing for viruses if you realy need it.
  • takeapieandrun
    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.
  • apache_lives
    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.
  • 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
  • Fip is right, cmon, it's not so easy to get a virus installed in your computer. Only real concern is about pendrive virus, otherwise, it's real hard get a virus actually, so just deactivate autorun and you're done.

    If you still think a Anti-virus is useful, look to the logs of your anti-virus, how much viruses have you executed? How much of these you wouldn't figured out even without any anti-virus? A computer virus it's not something invisible who will eat your computer's guts, it's just a program, and need to you to execute him at least one time.

    We aren't in 1996 with blaster or melissa. Tom's is supposed to be a site for tech guys, cmon, you aren't supposed to be a facebook brainless guy who don't know the difference betweek "naked gurls.exe" and "naked gurls.jpg".
  • 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.
  • Hupiscratch
    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.
  • cjl
    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.
  • 2 ddragoonss
    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
  • Manos
    Great article cause I was also of the opinion that "antivirus" crap mess up your computer's performance.

    My only dissapointement is that you dont show anything about Windows Defender which is free and convinient. Liked it more than AVG and the rest of the last crap i tried tbh. Now, as always I am not just virus free but also antivirus free cause I honestly dont like antivirus progs. I tend to format too often :)
  • karma831
    No MSE?
  • pandemonium_ctp
    Meh. Overrated and not conducive to gamers while in game. I'd like to see FPS differences while in games for this review to be noteworthy.

    I find active virus protection (resident shield) dropped me a few FPS - nothing major - but I don't need active protection while I'm in game; even if I do browse websites and multi-task. Also, file scanning while in game is really bad, regardless of the software or your computer's power. You always get lag - even if it's only a little - and it's always noticeable. I don't know how it affects SDD instead of HDD, but so far every HDD I've played on gets reduced framerates when the protection software is actively scanning files (and I use higher performance HDDs).

    yes - for inexperienced user. I have no problem with viruses over 8 years. OS Patches, working under different than admin account, not using IE, and not executing every garbage downloaded from internet. And finally virtustotal page for testing for viruses if you realy need it.

    Err, ya. Don't be so dismissively smug. I haven't had any major virus problems since Nimda was spread (and that was when I had Norton; big mistake trusting them): I update OS critical patches every 3-6 months (not very often); I've used my primary and sole user account (no seperation); I've used IE the entire time; obviously activeX and pop-ups are disabled.

    The whole hate for IE is ridiculous. I've seen as many problems with Firefox as IE. It's not 2002 any more. You're not cool because you use Firefox because it's "not as well known" or "not rampantly targeted by trojans". Those reasons are absolutely not true any more.

    I have resident shield off, no internet security, scan weekly with AVG Free (and usually cancel it after system files are scanned) and apparently I'm either extremely lucky or I just have a knack for not clicking where I shouldn't.

    Oh, Norton is garbage. Not only does it fight with Windows to run your system and interrupt kernel processes, their resolve processes are crap as well. Also, everyone I've known that used or uses Norton has mentioned problems with it, regardless of being savvy or not.
  • damianrobertjones
    Without reading: Yes, they do. Kasperksy Business edition on a single core machine. Oh yeah, it slows down, a lot