A few general thoughts:
1) Computer power supplies have limited extra current supply capacity. So adding extra electronics - e.g. extra memory to your computer without a larger capacity power supply can cause voltages to dip which can lead to problems with software crashing and resetting.
2) As I understand it, Windows Defender is the only virus protection you need. Defender is written by Microsoft themselves, who obviously have a vested interest in keeping their customers' computers healthy. Microsoft obviously know and have access to their entire Windows OS source code, so Defender is fully integrated and fully compatible. Updates for Defender are regularly published by Microsoft as new viruses and security threats are detected. Defender is free with legal copies of Windows OS, and can be configured to automatically update. So why use anything else ? I am told that Microsoft are not allowed to advertise Defender owing to competition rules ?
3) When computers have most of their memory filled, they can slow down significantly - even though there is some capacity left. To operate normally, computers shunt data in and out of their memory all the time, but when that memory is getting full, the computer can slow down and behave weirdly. I had this recently with an iMac, which although it had 10Gb of disc space left, was behaving very oddly and slowly - occasionally blanking the whole screen and not typing letters as I pressed the keys, but suddenly adding them after a couple of seconds delay.
4) Third party software can cause problems. Software that is available from the manufacturer's site will have been checked and vetted and approved by the manufacturer to work with their OS source code and integrate with their computers. 3rd party software won't necessarily have been through this process - especially if not downloaded from the Manufacturer's site.
4a) Third party software could contain malware and viruses hidden inside an innocent seeming application.
4b) Third party software might not be as robust as Manufacturer's Apps and might therefore allow hackers a 'way in' to your computer, even though the App they use to sneak in might itself be innocent and benign.