I want to ask one question I have an laptop with windows 8.1 pro and recently got the security intelligence update so shall I consider other after market AV or this one is effective.
I don't recommend using Windows Defender on windows 8.1 because it's not as strong as WD on windows 10 but it's more stable than the one on windows 10 and somehow consumes less resource
Windows 8.1 has Windows Smartscreen which is the best feature to protect you from malwares => Windows 8-10 users have significantly fewer infetions than Windows 7 users thanks to Smartscreen (Windows Defender also benefits from smartscreen)
You should use another AV such as Kaspersky, Emsisoft,...
However, every AV has some impact on your computer so you have to tweak your AV for better performance. I would not use any AV in its default settings unless they don't allow or they don't have options to improve performance
if you use Kaspersky security cloud free, you can visit my Configure thread to read my Kaspersky tweaks for maximum performance as a reference