Speed testing can be misleading. They’ll optimize the path to the nearest hosting site to minimize hops, giving you the best speed possible. Unfortunately, that doesn’t replicate the rest of the world. You may have 20+ hops between the source and your device, and the speed will only be as fast as the slowest node in that path. When streaming, the network protocols will try to reroute the data through faster nodes when it can, so you might see temporary slowdowns that last a minute or two. I see this once in a while where I live. Just remember, you can only control your equipment, your ISP can only control their equipment, and Microsoft can only control theirs. Everything else in the middle is a crapshoot.