It's as fast as a Muta, it will take more effort to kill than that.
I'm aware of the speed but if you meet it halfway you most likely can deter it before reaching it's target. If it's going to ignore the the patrol unit just reroute some air units to ambush it in between with it's weak stats it won't last long.
Economic damage is lasting damage.
Hardly it's comparable to taking your works off the line for a few seconds. The loss of workers is considerably more devastating because not only are the works taken off the line but you also have to spend extra resources to make new workers. Not to mention spend time rebuilding them this all adds up and is way more effective than a momentary halt of harvesting.