I've also tried copying the source to my own web server. Calculating the sum and pre-entering the value into the html. It recognises the correct answer but it still not fast enough, even though it's page load -> click submit. I've only tried with html, the original is a perl script so I'm guessing it notes the time before displaying the html to the browser.
I have a feeling the calculation is irrelevant (or at least in human terms).