With the recent update to version 4.15.0.28 it seems Garmin has managed to re-introduce this issue of massive lag on the My Day page caused by the live HR widget. Hiding the HR widget removes the lag, so that can be used as a workaround for now.
Thanks for the feedback. Maybe the issue is on my side then, since I’m currently on iOS12.1.2. I can see 12.1.4 is available, so I guess updating is worth a try.