I have recently installed Rockbox on my 60Gb iPod colour. I tested it with a few songs and it seemed to perform well. When I transferred the rest of my 55Gb music collection on to it performance dropped dramatically. Changing from one menu to the next or going back from playing a song took a long time, maybe 10/15 seconds, which is quite frustrating.
I'm not sure if this is something to do with my iPod or with the Rockbox firmware but just thought I would let you know my experiences.