I have created a dash app where there is a dcc.Graph input that renders an image from a dcc.Store on callback. The app works very smoothly and quickly for large images (numpy 3D RGB arrays) when the customdata graph property is None, However, when adding a customdata property to the graph to create a custom hover template, the app becomes much slower.
Inspection indicates that the actual speed of the callbacks is not affected (i.e. once the callback begins, it takes about the same amount of time). The problem is that once the graph has the customdata property, the callbacks take a lot longer to register (i.e. without the custom hover template, it takes less than a second for the callbacks to begin firing, but with the custom data, it can take between 5-10 seconds for the callback to register, and start processing).