https://www.forbes.com/sites/tedreed...s-this-summer/ - 6 years old, but covers the general concept.

In general using dogs speeds up the security process significantly - there's a short delay for the dog part of the process, but as you've said they then use metal detectors rather than full-body scanners which speeds things up.

Although I have seen this at SFO, in my experience it's not common there. I've experienced it at nearby SJC far more than SFO - I'm not sure if that's because SFO isn't TSA as such (but instead a third-party called CSA), or just because I've never managed to be there at the right time...