Hi,
Another solution to this could be using an ACF addon that adds a new type of Google Map field allowing you to add more than one marker to the Map field. That way, instead of loading 1 map field for every marker, you could have 1 map field for theoretically as many markers as you need. That should not only make your admin page load a lot quicker but also make your markers a lot more manageable.
https://wordpress.org/plugins/acf-google-map-field-multiple-markers
I know I’m a bit late to this thread but I hope this helps someone 🙂
