Hell,
You say in March 2018 that you are working on a new map:
"We are working on a new map. A different map is available at: http://maps.luftdaten.info/belgium/ , maybe this will get the new map soon."
I see the problem with this one: it's much too heavy to zoom in.
However, it's important to get the high resolution when zooming (until the basic OpenStreetMap).
I don't know what's the problem but would a solution be to go and take a new zoomed OpenStreetMap when needed on each zooming. Or to cut the big map in pieces and retrieve only the relevant part of the puzzle when zooming ?