Datapoint: I've recently spun up my mom's old G4 Powerbook 12" (running OS X 10.5 Leopard) to rip some old CDs I found, and everything was reasonably snappy. Well, ok, a bit slow, but fine, really. Until I opened Safari. After some 3 or 4 tabs the machine basically ground to a halt. (Fair enough: 256 MB RAM and a spinning hard disk!)

Things got a bit better when I switched off JavaScript. But yeah. The web is basically unusable on older machines. For many use cases all I want is some text, maybe some images, maybe some forms and buttons. How hard can it be to render that?