Just anecdotally I feel like I have to drive fairly aggressively to meet the estimates. I'm quick to disregard Waze because it will often throw me into an awkward intersection where I know taking a route with fewer turns would get me there faster. Waze also doesn't have speed data for many stretches of road. As it got added (and since) I haven't noticed any changes in routing or ETAs. I'd be very curious about an actual answer, but that's why I suspect it doesn't use it.