Chances are you probably know of this but I will go ahead and mention this anyway.
Press "Ctrl + C" as you have an item highlighted and it will clone them too, this way you don't need to go point and click on "clone" in the map maker menu every time you wish to make a duplicate. Way faster and efficient if you're just out of plain cheer objects everywhere. I find this being enough to get most of the tree placement work done when making maps.
As I believe Raz also told me a longer while ago that's not a completely bad idea is that you can actually load several meshes in object maker of the same tree and place them here and there, and then save them all as "One" object, this way you can load a pre-made pack of trees if that's what you're after. This is even better if you're after speed, but doing this will certainly not give the same detailed placement as a 1 by 1 kind of strategy.