The "new" reddit makes my 2015 macbook sound like its about to launch into space and the site really chugs in Firefox.

Old reddit is smooth as silk. I honestly don't understand why they changed it.

On my netbooks I have been using the html only version of Gmail because it became faster than the modern web app.

It's faster on my 2018 well-above-base-config Macbook, too. Somewhere along the way the original promise of this whole AJAX thing—faster updates, no page loads—got lost. The page loads are usually faster for real sites in the wild, where we have two versions to compare.

I use it everywhere now. It loads instantly, it's amazing. The only thing I miss is the auto categorization, I have to filter out all the newsletters with my brain now.

You can also setup filters which are easy to do in gmail and work better than autocategorization.

I'm tempted to make the switch myself, is there a way to permanently force this? Otherwise I'm just going to use Gmail from Thunderbird, which can be a Behemoth but it's not as damn slow as that UI at the end of the day.

This is off-topic but: reddit uses React to for building and rendering interface. Reddit is no.1 forum on the Internet,huge company with plenty of resources and top-notch engineers. So conclusion is simple and straightforward - it is not possible to write complex and at same time high performant app using React. Is there any mistake in my reasoning?

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.

Try https://reddit.premii.com/ It should work/load quickly on a 7-10 years old computer with the latest browser.

I have an older MacBook Air and reddit doesn't lag at all in Safari. Maybe it's something else..?