SiteSpeed follow-up: Add in backend time (TTFB) to the results
At first this didn't seem relevant to the SiteSpeed pipeline as we wanted it to be solely focused on browser rendering performance or frontend but in hindsight we will need to see how much time SiteSpeed waited for the data to be sent so we can effectively see if there's any performance issues in the browser specifically.
Task is to add in BackendTime to the results.