At the moment I am running Milky Way on three hosts, so I thought I would compare the runtime of the tasks to see what hardware is best suited to run the Milky Way application.

They all run XP
The host 857 is a Sempron 3000 that does a task in 530 sec.
The host 13208 is a P4 2650 MHz that does a task in 640 sec.
The host 13541 is an AMD Athlon 1200MHz that does a task in 850 sec.

So, what’s the limiting factor that favours my old Athlon in comparison to the newer hosts?

At the moment I am running Milky Way on three hosts, so I thought I would compare the runtime of the tasks to see what hardware is best suited to run the Milky Way application.

They all run XP
The host 857 is a Sempron 3000 that does a task in 530 sec.
The host 13208 is a P4 2650 MHz that does a task in 640 sec.
The host 13541 is an AMD Athlon 1200MHz that does a task in 850 sec.

So, what’s the limiting factor that favours my old Athlon in comparison to the newer hosts?

Maybe floating point. That and you're using more memory on the Sempron. Not sure.

Consider an upgrade.