Challenge: fastest program wins

Write a program in your language that
1.opens a file as arbitrary precision integer.
2.divides the number by 3.
3.stores the binary result in another file.

>>66
The point is to first know where is bottleneck and then optimize the bottleneck. Don't pre-optimize code on the basis of guessing where a bottleneck might be before you have even written some code; if you don't have an intimate knowledge of the problem domain, you should first write then profile the program. The optimization strategies I currently possess came about by first writing naive code then doing some deep analysis about what my naive code is actually doing.