While AVG continuously checks all running applications to make sure your computer is not infected, it typically doesn't slow down the computer. However, in case you encounter a noticeable slowdown after installation of AVG, please check the following.
Do you have some other anti-virus, anti-spyware or firewall application?
AVG can run alongside some security software, but it can slow down your computer and cause other problems as well. It is recommended to always use only one solution for each type of protection (viruses, spyware, firewall, on-line, etc.). See the article Using AVG together with other security software products for more details.
Did you install AVG only recently?
After installation, AVG runs an initial scan that will take longer than usual - sometimes even tens of minutes. Thanks to this scan, all following scans will be much faster and mostly will not slow your computer down. You can pause or stop this scan, or reduce it's priority by opening AVG, and clicking "Reports".
Is an AVG test or update in progress?
AVG test or update can sometimes slow your computer down temporarily - especially if a new large update is downloaded, or you have a lot of new files on your computer. Tests and updates are indicated by an animation of the system tray AVG icon. Or open AVG, and then click "Reports" to see all current and recent activity.
Does your computer meet recommended system requirements?
While AVG will run and protect you completely on a computer with minimum system requirements, recommended requirements or better are best for optimal performance. Check the article AVG system requirements and supported operating systems.
Do you suspect your computer is infected?
Some viruses or spyware can slow down your computer. If you suspect that your computer is infected, please refer to the article My computer is infected. What do I do now? for further instructions.