Also: If you call ToString on the StringBuilder in every iteration in the loop, that will force it to allocate a new buffer and copy the data to it when you continue to add data to it. You lose any benefits of using the StringBuilder, the performance gets as bad as using += on a string. So you will have shuffled around several gigabytes of data before you are done reading the 200 kilobytes. Despite everything, the person most likely to be fooling you next is yourself.