I also wrote an iOS app to do this exact same thing, although it’s unfinished and unpublished. I had a lot of trouble getting the app to perform well (what’s your secret?!)
Couple of things I implemented that you may want to consider:
1) onboarding flow to help users setup the app on first launch
2) filter certain workouts to include/exclude by type, duration, etc
3) home screen widget
Id love to chat if you’re open to it.
What I did is adding a loading indicator and fetching data onChange, so if I change e.g. bow heart rates are calculated it starts to fetch instantly after the change not only on the view with the bars.
And I'm currently just fetching 30 days. I think this will be interesting if I try to extend it.
Onboarding flow, Home screen widget is on my list. Thanks.
The filtering would also be interesting. Haven't though about hat because I usually myself do always the same workouts
The other big performance problem I've had is the simulator hanging when I try to use Swift Charts.
My primary interest was in monitoring my total zone 2 time for cardio workouts where I spend at least 30 consecutive minutes in zone 2. However I've reduced the scope for now while I get the minimum viable app up and running.