Maybe if you watch and actual garage door move up and down it's slow, because it's a huge object. But closer in with just a small oscillating system, it goes back and forth plenty fast. Fast enough that a bunch of guys not paying attention wouldn't have time to realize what's going on before they're hit.

There's even a mythbusters you can watch. Aside from the fact that breaking down something so minor is silly for calling an entire ending lame, the gun actually would work the way it was set up. So there's not really anything to complain about.