The modern web is an especially poor comparison. A big part of the reason why web works at all on modern computers is due to the intensive optimization efforts put into browsers, efforts that aren't backported to older operating systems or architectures. Chrome V8 will run on OS X 10.5 but not on PowerPC. So on a modern Macbook you'll get highly-optimized code from the JIT, and on an older G3 you'll get a much slower interpreter.
Native optimized apps that were made contemporaneously, like music studio software, are a different story. If it worked well on a G3 in 2000 it would still work well today. So maybe you have a bunch of plugins that you love using that you can't track down new copies of, or you don't want to pay for newer versions, or there's a computer in the corner with Gigasampler.
Music editing doesn't actually require that much processing power or I/O bandwidth, at least by modern standards. Quick back of the envelope math--your Pismo, if it runs at 400MHz, has a budget of ~200 cycles per byte to keep up with 16 tracks at 44.1kHz. That's easily doable, even if GMail is a mess.
Anecdotally, I remember looking at CPU utilization when I was working with DAWs back in 2000 or so, and it was never very high. You certainly can fill it with expensive plugins or huge stacks of oscillators, but even on a G3 there's a lot you can do.