Any program running on your computer will require a certain amount of processing power to work, which can affect your computer's overall performance. If an antivirus program is just running in the background, it shouldn't really have any effect on your computer's performance. However, when actively running a scan of your system, antivirus software can noticeably slow down your computer. If this is the case, try to schedule antivirus scans at night, or at a time when you're not using your computer.