Conditionally defer loading depending on wether a repeater is collapsed or not is not possible. At least not in any way I can figure out. Sorry 🙂
When we’ve had projects where we needed lots of maps/markers we built the architecture around that. So we created a custom post type which would each hold a single map (called Maps for example). Then in the relevant places we could just use a post field with multiple values or a relationship field to pull in all maps we wanted without performance issues 🙂
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