It's true: Antivirus programs do slow your computer, sometimes by quite a lot.

Credit: antoniodiaz/Shutterstock
(Image credit: antoniodiaz/Shutterstock)

Yet we've found several antivirus programs with system slowdowns so small that you'll never notice them unless you're doing something processor-intensive, such as playing a high-end game or rendering video.

The lightest paid antivirus program we tested is Bitdefender Total Security, which slowed our test laptop between 7.7 and 17 percent during active scans. Bitdefender is also one of our picks for best antivirus overall.

The lightest free antivirus program was AVG Free Antivirus, whose system impact ranged from 5 to 11 percent.

Passive slowdown Full-scan slowdown Quick-scan slowdown
AVG Free Antivirus 5.0% 11.0% 10.3%
Microsoft Windows Defender N/A 15.2% 14.7%
Bitdefender Antivirus Free Edition 5.0% 16.0% N/A
Bitdefender Total Security 8.0% 16.7% 7.7%
Kaspersky Total Security 12.1% 21.1% 14.3%
Panda Free Antivirus 7.2% 23.0% 7.4%
Trend Micro Maximum Security 3.4% 68.5% 36.8%

How we test AV software performance impact

We tested 12 free and paid Windows antivirus programs on an Asus X555LA notebook in the summer and fall of 2018. We used our home-grown OpenOffice benchmark test, which measures how long it takes the CPU to match 20,000 names with 20,000 addresses on a spreadsheet.

We measured system speed before any third-party antivirus program was installed; after an antivirus program was installed, but not during any active scans; during a full scan; and during a quick scan. Each test was run three times for each program, and the results averaged.

The numbers recorded were the completion times of the name-and-address matching tasks. Slowdowns were calculated using the pre-installation completion times as baselines and calculating the percentages by which each task increased the amount of time needed to complete the matching task. Baselines were taken separately for each program's set of tests.

The results

Neither Bitdefender nor AVG were the fastest in all three categories that we measured: passive system impact, full-scan system impact and quick-scan system impact. But both were in either first or second place for all three, while most other programs were less consistent in their rankings.

For example, Trend Micro Maximum Security beat Bitdefender, 3.4 percent to 8 percent, for smallest passive system impact, measured while the antivirus software was running but not actively scanning. However,Trend Micro's full scan hit our test laptop like a ton of bricks, slowing it down by 68 percent.

You'd probably notice that slowdown even if you were just checking email. You wouldn't notice Bitdefender's 17 percent hit unless you were intensively using the processor already.

Likewise, Panda Free Antivirus (7.4 percent) had less of a quick-scan slowdown than AVG (10.3 percent). But Panda's full-scan slowdown was in the middle at 23 percent, while AVG's was less than half that at 11 percent.

There was a runner-up among paid antivirus programs. Kaspersky Total Security came in third place in all three test categories, even as Bitdefender fought other programs for first.

There were a couple of free programs that did well, but for which we couldn't run all the tests.

Microsoft Windows Defender had the second smallest full-scan impact, and the third smallest quick-scan impact. But it's built into Windows 10 and automatically runs if no third-party antivirus program is present, so we couldn't really measure its background impact. We can safely assume it is less than most other programs' impact, though.

Bitdefender Antivirus Free Edition was second for both passive impact and full-scan impact, but it doesn't have a quick scan. AVG did tie or beat it in the other categories, though.

Most antivirus brands offer three or four different paid Windows programs, all sharing the same basic protections but adding a few more extra features with each increase in price.

For example, Bitdefender offers Bitdefender Antivirus Plus, Bitdefender Internet Security and Bitdefender Total Security, and each costs about $20 more per year than the previous one. But only the last has all the bells and whistles that Bitdefender offers, such as a password manager, webcam protection, parental controls or a system optimizer. When deciding which antivirus program to buy (or download for free), evaluate which one has the features you need.

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