McAfee antivirus software has been known to slow down some users' computers. In order for the applications to properly protect you from malicious software and other attacks, certain system resources must be used to ensure that protection is active and complete.
McAfee might be slowing down your computer because you have automatic scanning enabled. Scanning the computer for infections while you are trying to do other tasks could be too much for your system if you don't have enough memory or you have a slow processor.
Some antivirus/anti-malware programs offers an option to scan the computer only. McAfee, on the other hand, has options for active protection against threats. This means that the application starts up one or more processes to scan or shield the computer. Generally speaking, the more processes a computer is running, the slower it will be.
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As of 2010, hard drive technology simply can't allow a full scan to take only a few seconds. Individually checking each file on the disk for infection takes time, and the older/slower your hard drive, the longer it will take. A full scan is also the most resource-intensive task that McAfee can do, so expect other applications to run more slowly.
If your McAfee software is set up to automatically update, you may notice this slowing down your computer or Internet connection. Your connection may be especially slow if you are using a dial-up connection.
Perhaps your computer does not meet the minimum system requirements for McAfee software. Check the McAfee website for information regarding the minimum system requirements for each of their products: http://home.mcafee.com.
If you have multiple security programs installed on the same system, you may run into problems. This is a result of the applications not trusting each other and fighting for permissions. In general, have only one antivirus program installed at a time.