Not intending to defend react, but you are neglecting other possibilities such as that they have not tried, tried too little or that it's not important to their business and others targeting different userbases could succeed.

Without a definition for "high performance" the statement doesn't really say much. If we had a definition for it, we'd also have to ask if "high performance" was even a goal for the reddit team. As you stated, reddit is the most popular forum on the internet and it has continued to grow even with the new design so perhaps the performance concerns were considered premature. I know some would disagree based on their own issues with performance, but it's at least safe to say that most people using reddit don't seem to have a problem with it (if there is one thing redditers love its to complain loudly about why reddit is going to shit)

Yes, your sample size is one.

Netlify is also built with React and its homepage loads amazingly fast (<1MB in size).

Are you sure that Netlify homepage uses react? I have react developer tools installed and it indicates no react presence on netlify's (very plain and simple - I mentioned complex apps in my initial message, but that is another thing) homepage.

Another example could be Facebook which qualifies into "complex apps" category and at the same time is very well known for sluggish performance.

Not sure if that's a typo and you meant Netflix or not, but did you know that Netflix actually gave a presentation about removing React from their homepage, and got an incredible performance boost from it? I suspect something similar in the case of Netlify, if you're referring to them.