Timeline on iphone vs ipad ; iphone is buggy?

On my iphone, ios15, the timeline view is very buggy. When I swipe to a motion event I get a spinning icon which doesnt stop, and I would be mistaken for thinking this is an internet problem, but .. if I hit pause then play it works immediately!. Swiping through the timeline is not paying videos properly I have to do the same trick all the time, pause then play. Also see below I do not get the lo-res version when I swipe. Same for multiple devices. In stark contrast.... the timeline on my ipad is brilliant. As I swipe through motion events I even get a lo-res speedy version of the recording - I have not seen this on my iphone? My ipad is on ios14 but is less powerful than my iphone. Why aren't these the same experience? Is there a bug on ios15? Iphone is an Xs. thanks