I have been using Avast free since years. Since the beginning it runs pretty light on the system, and never had slowdown issues because of it. It still runs well. If someone says that it made their PC slow, then they need to look for their configuration or some other problems.
Windows Defender makes my PC really slow, and I hate that. With new versions of Avast, I uninstall it and then install it from setup. When I uninstall Avast, Windows Defender kicks in, and my PC runs so slow with it. It boots up slow, and if you open a folder full of exes, then it will take a long time for the folder to stabilize, and show the icons.
Windows Defender can be considered a good option for general or novice users, where they just need an antivirus, and they do not have to deal with it much.
But, Defender does lack few things. Avast has web shield, which protects from malicious sites, and it has often saved me and alerted me whenever I visited any such site by mistake. Mind you, I am very careful about opening unknown sites and I observe safe practice... but being a site and forum moderator, sometimes I do have to follow links. Avast has saved me with its web shield, and I think it is a very important component. Don't feel safe without it.
Third party antivirus do offer extra protection from stuff that Defender lacks, and so an advanced or experienced user, will of course prefer the third party antivirus.
Of course, first thing is the common sense and observing safe practices, but still extra protection is always a good thing.