6 years later, I'm running into a very similar issue on iOS 15.2.1 with Google Maps API on a map with lots of distinct markers. (I'm not using markerwithlabel however, but custom icon markers - I need the markers to be different colors) Works 100% on Chrome on Windows; but in Safari on iOS, you can play with the map a little and then it freezes up. Any updates?

I'm waiting for PWA push notifications on iOS as well - when will Apple support PWA to the same extent that Android does...