Google Maps assumes that you are being a good driver and going no higher than the posted speed limit, and slower if necessary based on the speed of traffic.
When you input your destination into Google Maps your original estimate is made based upon posted speed limits, current traffic conditions, and the distance between your starting point and destination.
Waze does not take into account the speed limit and only looks at the speed of traffic. U.S. drivers tend to drive 15-20 mph over the speed limit on expressways (source - personal experience), and Waze assumes you will drive the speed of traffic. Note that driving over the speed limit is (obviously) illegal, so break the law at your own peril - it's not Waze's fault if you get a speeding ticket.
Here's an Medium article written by Waze themselves explaining how they compute travel time. Notice that the article does not mention speed limit as a factor.
I have taken long (4-5 hour) trips using Waze. I always drive the speed limit, which is often significantly slower than the speed of traffic, and I usually end up spending about 25% more time on the road than Waze predicts.