Storing it in large bitmap won't speed things much. You'd better order up rendering calls so you first draw all tiles which are the same image / pattern. i.e. if you have a field of (1,1), (2,1), (1,2), (2,2) and the first and the last tiles are the same image (preferably pattern too), you'd first draw these tiles. PowerDraw does buffering which accelerates things.
- Lifepower