Risike has made a Lua script that tests the performance of Micro DS lua.
It will draw a lot of bounding an rotating balls with different sizes of both screens. After 1'20'' it will display the max number of balls it can display without fps fall.
I tested on no $ gba and I found 58 bullets. I have not yet tested on a real DS. In any case can be moved on display 58 images by all possible operations above (change of color, rotation and zoom), which seems quite reasonable given that one can display Maps almost as quickly as it displays an image.
