i had a 10.3 machine not long ago. an ibook g3. was pretty good on safari. the only thing was even with my 50mb connection at home and even faster one in uni/work (research student, uni is my work too) it did not render the pages properly and was still slow. it could just be the internet has moved on from our machines?
i remember when i did have the ibook i actually used (now dont kill me) internet explorer for mac. it seemed a lot better, in my short time using it, at rendering the pages than the older safari.
i don't think it was a problem with the machine as i re-installed the OS again so it was at its factory state.