While I do agree that the modern web certainly is bloated, I see more than that at play here. I think what's holding back PPC macs in the modern era is the fact that they were already slightly underpowered, even for their time. A PPC G3 iBook from 2003, the last year they were made matched up against a Pentium M laptop from the same year leaves the Pentium M running circles around the G3. It does the same with the G4 laptops of the time. This gap only grew wider and wider as time went on. By the time the Intel transition happened, the most powerful G4 laptops - the 1.67 G4s matched up against the 1.83 or 2.0 CoreDuos - weren't even a challenge anymore. I have a 1.83 CoreDuo from 2006 still running the modern web just fine, still plays youtube in HD, still snappy as ever. The 1.67 G4s? They can manage but struggle and the struggle is getting more pronounced as time goes on.
Some browsers allow you to set the useragent to something Mobile so it reports to the websites as if you're using an iPhone or Android device. I've found that this helps a lot and even an old iBook from 2001 can still very much handle the modern web as a "mobile" device. Are there any broswers for the mac that allow you to do that? I'm not sure TFF does, but if it did that's one possible solution to the lagging PPC problems. Most people already have some sort of smartphone anyway so it really isn't a change when using the mobile versions of websites on a computer, all the essentials are there without the bloat.