I've never seen this issue myself. I'm on High Sierra now.

In Yosemite PDF rendering performance was crap for me in some high resolution documents for print, but other than that it has been flawless for as long as I've been using macOS.

I've only seen the issue shown in the Reddit post (which looks like some rendering lag) in Android actually.

As I said it's difficult to see on a high PPI screen unless you're looking for it. High Sierra, I think, is the first version where this bug was introduced, so you most certainly have it but it just isn't drastic enough a change to bother most people on a Macbook's screen. The rendering lag is another problem that was introduced with the one I'm referring to, which is a separate issue but they are probably related.

I have to admit I haven't used a low DPI screen in macOS since Maverick times, so you might be right. PDFs in Preview look perfectly fine to me.

Maybe Apple actually optimized rendering for hiDPI displays since probably that's where the majority of users are these days.

Oh wow, that definitely looks terrible. I totally understand not liking MacOS due to that. Thankfully it's much less pronounced on high DPI screens? I've never owned a low DPI macbook so I've never really encountered it before.