Since file operations are a relatively slow process and you have the file operations embedded in the loops with the rest of your code, the file operations are slowing down your while loop.
What you should look at is the Producer/consumer architecture (search the forums, and go to New... in the File menu to find a template for it) so that the data you want to write to a file is passed to another loop by way of queue. That parallel while loop dequeues the data and writes it to a file at its own pace letting the producer loops continue with there activities at their pace.
I won't comment on the unneeded use of local variables and the flat sequence structure that don't look necessary either.
