preview the map by loading only the top blocks. changing a chunk will cause it to be fully loaded or just as superficial

What exactly do you mean? Can you elaborate on this feature request a bit more? Thanks!

full loading of the map is too long, sometimes you just need to see how it will look from above and quickly

Use /dmap to delete the maps you don't want, and the set the perspective for the (flat) map you do to a low res choice (like iso_N_90_vlowres)

https://github.com/webbukkit/dynmap/wiki/Configuring-Maps-and-Worlds-using-dmap

full loading of the map is too long, sometimes you just need to see how it will look from above and quickly

You can also speed map loading via storing map files on an SSD, increasing bandwidth between client and server, and using an external webserver like apache and HTTP/2 or HTTP/3 protocols