Is GarageBand the only application that is slow, or is your mac also sluggish with other applications?
The first thing to check is, if you are running out of disk space on your system drive. If you let the available space drop below 10 GB, your Mac will become very slow.
See these general hints: Mac troubleshooting: What to do when your computer is too slow ...
Does the problem only happen with one project or always?
Does it only occur, when working from a different user account?