WebEngine performance incredibly poor

I am using the WebEngine examples in the version of Qt Creator that comes with 5.5.0 RC but I have found the same issue at least as far back as Qt 5.4.2. For example, when I build the WebEngine Tab Browser Example for Release and run it, it loads any page extremely slowly. The main problem for us is that our sofware needs excellent integration with Google Maps but this is one of the sites that works especially poorly. Once you type in http://maps.google.com.au you can wait minutes before the entire map appears and then when you zoom in, it first does a bitmap scaling and then a few minutes later it catches up and fills in the details properly. I have attached an image of Google Maps about 4 minutes after I zoomed in, still showing the pixelated view and also waiting for it to switch to Satellite mode. Panning is also incredibly slow with the map rendering in sub-squares with each region taking a minute or more to load. By contrast, all the web sites which perform poorly with WebEngine (which is basically everyone) load almost instantly on native browsers like Chrome, Firefox and even IE on the same machine.

What you see is very similar to the days when I had an old & slow dial-up connection so perhaps it has something to do with comms (I don't know). The actual internet connection is a 120Mbps cable connection where other browsers almost never show much of a delay in loading any page on any website.

I have also attached the qtdiag output.