Why are browsers so slow?

The OP isn't talking about rendering (at least as I understand it, which isn't well either), it's talking about why, after rendering, it takes so long to switch between tabs or why creating a new clean tab takes more than a millisecond.

I, like the OP, don't understand why that part is hard. Does it have to rerun and redownload all the javascript each time you switch to a tab? Are we misunderstanding how websites work? Will removing all JS solve the behavior the author mentions? Obviously doing that will make pages load faster. But that's not what they seem to be asking. Is the OP question not meaningful because of some aspect of page rendering?

Honest questions, I have basically no knowledge about this topic, but have wondered the same thing.

I only notice the kind of "tabs are slow to open" behavior pretty randomly, and without knowing the state of the rest of my machine at those moments (is there something else pegging a few of my cores for some reason?) I can't confidently place any blame on the browser itself.

For what they are, I consider modern web browsers to be exceptionally fast and reliable, and they tend to put a lot of other applications to absolute shame. They are imperfect, but still very, very good.

Fully parallelized browser using something like Servo and Vulkan for rendering graphics is your friend.

And immediately any webGL visualizations / games / necessarily resource intensive webapps are rendered useless as they're automatically throttled back by the browser.

> But I remember the times when we had the amazing Opera browser. In Opera, I could have a hundred open tabs, and it didn’t care, it worked incredibly fast on the hardware of its era, useless today. You may ask: why would a sane person want a hundred open tabs, how would you even manage that? Well, Opera has had a great UI for that, which nobody has ever matched. Working with a hundred tabs in Opera was much easier back then than working with ten in today’s Safari or Chrome. But that’s a whole different story. What would you do today if you opened a link and saw a long article which you don’t have time to read right now, but want to read later? You would save a link and close the tab. But when your browser is fast, you just don’t tend to close tabs which you haven’t dealt with. In Opera, I would let tabs stay open for months without having any impact on my machine’s performance.

This is exactly how I'm using Firefox, right now -- 273 tabs open. It's sitting at ~5% of CPU and 450MB of RAM. And switching tabs is instantaneous. And it doesn't reload every page when I quit and restart my browser -- it only loads a tab when I click into that tab. And it has tab groups so that my current group has only about 8 tabs in it, and the other groups are sorted by topic. And on and on.

I find it frustrating when people post these articles as if everyone has this problem, and don't provide enough details as to their setup so that people can help them fix it. I promise you -- if everyone around the world right now had the problem the author was having, it would have been solved. No one would stand for it. Rather than assume everyone is suffering just like you, assume that other people either a) don't behave the way you do, or b) have found a way to fix the problem.