virat8586 :
If you have an original windows...you don't need any antivirus. As it has an inbuilt antivirus "Windows defender"
Apart from that every other anti virus will slow your system down
If the built-in Windows Defender would do excellent job then there wouldn't be any other AV programs around. But since it's not the case (Win Defender is one of the worst), there's plenty of market for AV programs.
Also, not all AV programs will slow down the system.
Here's 2 days old article that covers the latest flaw in Win Defender,
link: https://arstechnica.com/information-technology/2017/05/windows-defender-nscript-remote-vulnerability/