Replace DivIcon thumbnail markers with lightweight CircleMarkers.
Each DivIcon created a DOM element with an <img> tag, so 3K+
geotagged photos meant 3K DOM nodes and 3K thumbnail requests
hitting simultaneously — freezing the browser during clustering.
CircleMarkers are SVG-rendered on Leaflet's canvas layer with no
DOM nodes per marker. Photos still open in preview on click.
Also: bump cluster radius 50→80, enable removeOutsideVisibleBounds,
disable clustering at max zoom, increase staleTime to 5 min.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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