I realize Google is doing an internet search, but the point is, it DOES the internet search! Siri stops, waits, and asks if I want to do an internet search rather than just providing the answer. So it's another step (yes or button press) to continue. Again, contributing to my point of the speed of the Google app.
I don't disagree the Google app is faster. There are some things I really like about the Google app (speed is a big one).
I like that Siri has Wolfram Alpha results built in because it expands the search possibilities immensely. Sometimes you have to explicity tell it to use Wolfram, but I've found most of the time you can just do a query and it'll check automatically.
Because of the WA integration, I can ask Siri things like "How long would it take to drive 600 miles at 55mph?" (GVS returns a bunch of random search results, the first of which is some algebra word problem site).
Or I can say, "How much fat is in 3 eggs and 2 slices of bacon?" (GVS again returns a bunch of irrelevent web results)
etc.