If you're running a heavy desktop environment like KDE it will slow down an older machine.
You might try Fluxbox for a windows mgr on an older machine, or something similar.
Adding memory to any machine will perk it up if you only have 128MB or less in it. Linux swaps just fine but swapping to HD slows thing down. 256MB or more will perk a Linux box up.
The OS you are using also plays a part in it. You may find that Slackware will run apps and processes faster than say Fedora.
In my opinion there is no better OS to run on old hardware than FreeBSD. Slack works ok too though.
And if you are using a P133 then it's a P133 and thats it.
I use a PII 400 sometimes and it will peg 100% while compiling or ripping. Your interface with the HD, the speed, will affect writing. That old HD on that PII is only running at UDMA33, about 14MB sec. Only has a 40 conductor cable on it. You'll need an 80 conductor cable to run UDMA66, UDMA100.
Also if its an old AMD K62 type then it is just slow. They were slow. Something to do with floating point interger problems.