Reasons for High CPU Usage

When your computer seems to be running slower than usual, you can open the Windows Task Manager to find what's causing your computer's CPU usage to spike. Generally, high CPU usage is caused by something innocuous, such as a background operating system process. However, in some cases it can indicate a more serious problem.

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Many modern processors can run at speeds of 2 GHz or more.
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Background Programs

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Background programs
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If you have several programs running in the background -- particularly on an older or lower-speed computer -- you may experience temporary periods of high CPU usage as those programs become active. For example, many programs have background components that periodically search the Internet for updates and download them automatically. File-sharing software may also cause high CPU usage while downloading and uploading data.

Malicious Software

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Malicious software
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Malicious software can cause a computer to behave in abnormal ways. Some malicious software may utilize the Internet connection of the infected computer to search online for other computers to infect, or may modify files on the host computer. Always use a trusted antivirus program to reduce the possibility of a virus infecting your computer.

Web Browser Plugins

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Web browser plugins
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Some Web browser plugins can cause high CPU usage while they are active. This is particularly true of the Adobe Flash Player plugin used by many streaming video websites. While most computers can play a single standard-definition video in the Web browser without difficulty, you may experience high CPU usage when playing high-definition video content or if you have multiple video player windows open simultaneously.

Crashing Program

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Save your open work
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A program can cause high CPU usage if it becomes unstable and begins to crash. In many cases, Windows is able to detect an unstable program and will ask if you want to close it. If this does not happen, you can often resolve the issue by ending the program via the Task Manager. Save your open work, if possible, before doing this.

Windows Process

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Windows process
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Windows has certain background processes that may run periodically as you use the computer. For example, the Indexing service scans the hard drive for new files and adds them to the index for faster searching. You may also have Windows configured to download updates automatically from Microsoft. These services may cause temporary increases in CPU usage, which generally resolve within seconds or minutes. If you have not configured Windows to update itself automatically, run the Windows Update application periodically and download fixes that may resolve issues with Windows that could cause CPU usage spikes.

CPU Heat

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CPU heat
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Modern computer processors have the ability to lower their clock speed when they are close to overheating. If the CPU in your computer is operating in a speed-throttled state, normal programs will consume much more of its resources. Make sure that your computer's fan works properly, and that none of the intake or exhaust vents are blocked.