Ok so I've done the test a bit better this time. I made sure that I rebooted and had nothing running and I set the performance at maximum - same on both Mac and Windows versions.
This is on a Mac Mini Dual Core 1.66, 1GB Ram.
With 10.4.8 using a 5400 rpm internal drive scratch, I get: 5:10, 5:01
With 10.4.8 using a 7200 rpm FW400 drive scratch, I get: 4:37, 4:32
With Windows XP SP2 on the same machine with the 5400 internal, I get: 12:18, 11:49
wtf is wrong with the Windows version (or Windows itself perhaps)? It doesn't even show the image in the back when the thing is running. The whole image goes grey and white squares.
I couldn't test Windows with the external drive because it's HFS+.