Off the top of my head (and not being a Mac guy) I'd suspect that whatever things you had on your laptop that made you dissatisfied with it are now running happily (and slowly) on your iMac.

Before I would do what is effectively a clone of one computer to another, I'd make sure the source computer was all squeaky clean. Otherwise, you're just inheriting whatever creepies and uglies were in the source computer.

I know - Mac's are immune from virii (as we've all been told so many times here ) but still ...

If it were me, I'd have just transferred the data files from the external hard drive, and re-install any programs that were necessary.