Final Cut Express will be miles better than iMovie in terms of functionality, although this will come with increased complexity. It'll take a while to master.
It shares the UI with Final Cut Pro, although has many of the features removed.
I doubt it'll help though if a speed increase is what you're looking for. You'll lose a lot, if not all, of the one-click functionality. It pretty much only supports DV as input format (well it did last time I used it), so you'll have trouble using it with most modern consumer-grade camcorders.
What exactly is sluggish about iMovie for you at the moment?