Most likely just the ravages of old age and time slowing things down, bad sectors being skipped across and worked around. Laptops get a hard time with vibration and being knocked about etc and the HD suffers most when this happens whilst its running. Have put up in the past with a drive with about 25% bad sectors before replacing it, now that did get slow. Trying an error check (with 'scan and attempt recovery of bad sectors') before your next defrag may help. If not then try a format and software rebuild from scratch, sometimes the disk becomes to fragmented to be usefully defragged.
Laptop drives have got faster over the years and are available cheaply now. They simply slide out and back in so easy to replace. The problem you may face is that the bios of an old machine like you describe may not have large disk support making it unable to 'see' even the smallest disks available these days and finding a bios update for an old machine can be tricky.
If its still a useful tool you may, I'm afraid, just have to live with it and accept that its not going to be as fast as modern stuff.
Oh one last thought, if all else fails more memory would help, cheap as chips now and takes the load off the HD to some extent.