Just in case someone else has this problem:
My mini was very slow with SL and crashed a lot but I put up with it for ages. I decided to reinstall it and needed to use my 8Gb SD card as the install disk. I had moved my home folder to the SD card so I needed to move it back to the main drive so I could format the SD.
As soon as I moved it back the system was lightning fast. I am convinced that having my home folder on the disk was causing the slow down. I didn't think it would because its just my files on there, but the home folder probably has all sorts of stuff necessary for the OS to function. I am going to reinstall anyway but this time I am only going to move my iphoto gallery to the SD as that is what takes up all my space.