Drupal Anonymous Commenting

We are looking to provide Anonymous Commenting option on our website. Currently only Authenticated Users can comment. Now question is, how to fight the spam. We felt Captcha will not be the right option. If we use Captcha, Drupal Caching will be disabled and we don't want that. To keep the Caching intact, we can load Captcha using Ajax, when user clicks on Comment field. But we don't want to reload Captcha using Ajax (which is what offered by Boost Captcha module). We are already using HoneyPot Type modules which inserts a Hidden Field in form, but it is not sufficient. Check the below given Times of India link in which Captcha is appearing on clicking on "Full name" field in Comment form, without using Ajax. We are fine with Captcha, if it wont disable Drupal Caching and if it wont load using Ajax, it should appear instantly on click on Comment field, like happening in below Times of India Link. [login to view URL] We need, you Drupal expert views and effective implementation without compromising on user experience and site speed.