My apologies for firing off term papers in your direction tonight, but, here goes:
Core 2 Quad QX9300 and 8GB RAM running Windows 10 and Edge/Chrome and uBlock Origin.
Last year, before its "the quadro version of the GeForce 8400" died, I was playing around on a similar system. Core2Quad Q6600, 4GB DDR2, aforementioned GPU, SSD, 10, Chredge, uBO. (*base machine Dell Precision T3400, 2007).
It was
fine. It didn't rocktane/shocktane set my world on fire but if I'd upgraded it to the platform-with-bios-updates max of 16 gigs of RAM and put a fresh video card in, it would have been able to substitute for my own daily driver, then an i5-2300/gtx750ti/16gb/ssd, just fine. It's possible I was giving it more benefit of the doubt because it's from 2007 and I'm generally super impressed with the way Windows 10 worked very well on extremely old hardware (it's not fundamentally any heavier than Vista is and the only reason it doesn't run on PIIIs is because Microsoft added some requirements for a couple CPU instructions).
I ran a couple web apps (onedrive.com, word, onedrive) in Chredge, did my normal social media load on it, listened to spotify on their web site, watched a couple youtubes, used discord and visual studio code on it and it handled the load fine.
If I had bothered to use native or non-electron solutions for all those things, it would have left more load available for casual web consumption, and the whole experience would have been that much more "fine".
So, I 100% agree with your assessment that while the web is wild and it is a performance driver in the modern context, it's not quite as dire as some, even me in some contexts, make it out to be.
But, it's worth noting that even one thread of 65nm Core2Duo is like... 4x as fast as a thread of G4, absolute minimum, plus 64-bit, plus chromium having a couple efficiency/reliability things done that firefox didn't yet have at the time, etc etc. And that's all on top of "web browsers can run individual tabs in their own threads on their own CPU cores, now" trick that started happening a couple years ago.
I may be wrong but I think the future of browsers on older platforms is probably going to look more like new web engines (and accompanying JS engines) written specifically for use on systems with limited resources, wrapped in native platform chrome.
This would be super interesting to see. I don't agree with the way Cameron Kaiser framed both Classilla and TenFourFox, and some of the decisions he made such as explicitly excluding Intel users (even excluding Rosetta) but I get the idea and the intent: both in terms of extending the life of the machines and in terms of "make using the macgarden better" -- which, tenfourfox in particular
does do.
To your point about javascript behemoths - I don't think we can ever expect any PowerPC Mac (even a dual 2GHz PowerMac G4 with 2 gigs of RAM, or even The Quad with a fully kitted out 16 gigs) to do well with some aspects of the modern web. From a "vintage computing" standpoint, I'm not bothered by this. Slack, Discord, and the OneNote Web App didn't exist when these machines were shipping. Most of them didn't even exist until after Snow Leopard, the first version of OS X not to run on PPC Macs, shipped. Most of these things didn't even exist until OS X 10.5.x officially fell out of Apple's "support" lifecycle, upon the introduction of 10.7.x in 2011.
Anyway... it would be
super interesting to see what a modern browser engine could do on that hardware. I'm probably going to pull out my iBook G4, update TenFourFox and just poke around. But, I'm also kind of in the camp of... "is it necessary?" - MacGarden appears to be in the process of bringing up an updated version of their site with the explicit intent to be friendly to older computers.
This... I'm just thinking out loud here. This whole situation is going to be interesting over the next couple years as people really do get more into XP/OSX-era and newer "vintage" computing. The Web, genuinely, is a meaningfully big part of this era. But, unlike a copy of Word, Photoshop, or Myst -- the web is not, itself, static.
For newer computers from time periods where the web was a meaningful and important part of the overall experience, it's going to be super interesting to see how people handle things.
And, again, I think it depends on whether you view access to the web as a tool to enhance the rest of the machine (e.g. access reference info about period software) or an integral part of the experience (e.g. read blog posts and write forum comments.)
It's tough and I don't know the answer. I don't even know what I'll think in three years.