I did a test between two devices, one device is rated 30 fps and the other one is rated 60 fps. Everytime I tried to watch HD content on the 30 fps device, there was crazy lag but when I watch the same HD content on the 60 fps device, there was no lag and the content ran buttery smooth.
Whatever you guys did to the app, has crippled devices with lower fps to the point HD content is no longer watchable. This issue needs to be fixed either by coding in a device detector that will force the app to play HD content at the appropriate fps for said device or roll back the update until you guys have figured out a solution because the last update that was pushed for the TV Stream app, still didn't address this issue for a lot of devices.
Until then, fellow customers that are still experiencing the lag, "DO NOT WATCH CONTENT IN HD!". Choose the watch option that DOESN'T say HD and things should be back to normal again other than the fact that you'll have to start putting your content in fullscreen manually now.
what device are you playing the xfinity stream TV app?