Probably because the actual program is stored on a remote server (I think). The data has to be sent from your computer, through the internet, processed by the program, and sent back. The entire page also has to be refreshed to show the new data, so that takes time too. On top of that, there is probably several people using the calculator at the same time.Can you explain then why Metalkid's takes about a minute? I don't know a lot about computers, but if his takes a while what makes you think mine won't?
If you download the offline version of MKPPv4, the damage calculator on that is almost instantaneous.