IIS website slow in response when calculation in progress

I have an IIS website which connects to MySQL. I have a link in the website which do the intensive calculation. When this link is triggered, it uses around 20% of CPU resource. Then, if I click other links, the other links are responding very slow. Until the link that does calculation finished, then other links responding time will resume. I am curious why only 20% CPU is used, but the other links still respond slowly. It seems the calculation link occupies the CPU resource. But isn't it different link are run in parallel, how it should be affected by intensive calculation of one link? How can I improve that, or should I tune something in IIS of my code?