I got some feedback from Jeroen Van Den Berghe that when he tried out our dashboard (he also use WebPageTest) that the numbers when we compare todays SpeedIndex to one week/month back seemed to be wrong (by looking at the actual values instead of just compare) in https://grafana.wikimedia.org/dashboard/db/webpagetest
Lets verify and test that they are ok + also check at the same for our Navigation Timing metrics.