I guess those who don't feel any system impact should not believe you either then. Any AV will have some sort of impact regardless. I've seen people say they found Kaspersky and Avast heavy, so I guess by your standards they are wrong. In all honesty everyone will have different experiences and that goes for any AV. You've given some good examples where WD may be slower than others, but I would argue that the vast majority of users probably don't have a downloads folder full of exe's and aren't compiling code, so in many respects WD will be plenty fast for them. I've used WD on and off and to be brutally honest, more often than not, I actually find it faster than most 3rd parties. Keep in mind though, I don't have a folder full of exe's and I don't compile code, so in my instance I don't even notice it. That doesn't mean it will be the same for someone else. At the end of the day, everyone should use what works for them, but I wouldn't be ignoring, or implying that others that have a different experience than you are wrong, lying or making it up.
I have multiple computers also, and my experiences are the same across all my devices. In all fairness though, performance has a much bigger picture than compiling code and opening a folder full of exe's. To me when I hear someone say it has a massive system impact, I'm thinking, booting into Windows, opening programs, surfing, etc... I'm not thinking of a few select tasks that is not typically common place amongst all users. If your compiling code and have a folder full of exe's across all your devices, then yes, your experience will be the same. As I've said above use what works for you and fits they way you use your computer. The same can be said for everyone else, so if someone uses WD and they like it and they don't have any issues with performance, then that's fine, but I wouldn't discount their opinion just because it's different from yours.