Flash doesn't render tiles very fast. It's great at storing graphics that don't change in a buffer and quickly displaying this buffer quickly. Every graphics object (or sprite) that is added as a child to the stage has to be rendered independently.
Your problem is flash has to draw 50x50 = 2500 tiles every frame! Even if they 3x3 pixels, flash still treats them as separate objects.
It would be nice if you could store everything in a buffer or in one object and display it. So try drawing these 3x3 tiles in the same object instead of multiple objects. However this might hinder your functionality as the whole object will have to be re-rendered every frame.
Some other suggestions might be to render the objects in larger cells on a grid. Instead of having 50x50, start with 10x10 and then draw 5x5 cells in each cell. This would probably speed things up.
Hope this helps. If you find a good solution please post!