Not sure if it's because I have pretty strong computer (40 cores, 64gb ram, ssd), but while I don't experience debilitating freezes (20 minutes), it can open up to a minute each time I want to add new map. I don't have Slate open, the basic map editor does this just fine for me.
It's true I do have plugins, but only minimal amount.
One think I suspected before, is when I open map/browser, all the tabs inside are open as well, including all the "scene materials" and editor materials, and they start instantly updating. If I close those tabs, the map/browser is fast(remember, it's map/browser thing which is slow, not the editor !). I just can't force 3dsMax to remember having those tabs open. The whole updating can take a bit since by this time I can often have over 1000 materials in scene (blame decos...).