Map size and lag

Just a quick question. At what file size do you find that maps lag roll20 to a point where it's no longer acceptable? 1mb, 2mb, 5mb?

I prefer to keep my maps under 1mb to 2mb so I have wiggle room for dynamic lighting, objects, tokens, and such.

In some of the packs I've created, I've attempted to include both the tile pieces but also some premade base maps from which GMs can further build upon.  The basic idea being that rather than have the GM put up sixteen tiles to create the base terrain which may itself not be overly complex or critical to the game in appearance (versus the buildings or features that appear on top of that terrain), they can instead use a large 20x20 or 25x25 square base map that is less than 5mb to save on rendering times.