Roll20 has made some pretty sweet advances in larger maps. Earlier this year they changed the loading scheme. Roll20 automatically makes and loads sub-versions of your big map background graphic. I don't know if I can explain it properly but basically if you drop a large map image (say a JPEG), when you zoom in at certain levels it only loads a portion of the JPEG. When you zoom quite far out I believe it loads a version of the whole image that's appropriate for screen size. Considering this feature, I think that the best practices approach is: make your whole map as 1 piece in 1 JPEG-format image (except for moving parts like character tokens or a treasure chest perhaps, that could be added on top in-game). By the way I haven't had any problems with large maps loading even on a cheap laptop several years old, since they upgraded this feature some months ago.
So I would say yes, dropping large quantity of objects on the screen is something that could slow it down, on perhaps a slower computer. Note that PNG images take longer to process, JPEG images are faster to process. PNG images can have transparency which is nice, but at a cost if you have a lot of them.
Not sure if drawings from the drawing tools could ever slow it down. I think it might slow it down if you put a lot, like tons of walls and barrels and rivers all across 100x100 just drawn with the tools in Roll20. At that point I think it would be good to erase some of the lines in the sections of the map that you aren't using, or draw it in a graphics program and save-it down as one big JPEG.
