Browsing suggests both Sky Go and the BT Sport app share some of the same rechnologies as there are certain reoccuring names in terms of the software developers and live streaming third parties. So the issues I'm experiencing with the framerate on both apps for live broadcasts may not simply be down to the apps themselves but the content provider(s). Since recent updates of both apps, comments are eerily similar in terms of the issues being experienced.
It's all very curious.