Disqus doesn't slow down page load times because we use a non-blocking technique to load required files. We work hard to make sure that we don't add any performance regressions to sites using Disqus.
We are also working on making comments made via our product indexable by search engines.
As for the open source version of Disqus, I am not sure what you mean by that since Disqus is a service and not a program that you can download and install. People prefer Disqus because they don't want to spend time implementing and hosting discussion platforms for their websites just like people using GitHub don't want to spend time implementing and hosting developer collaboration software.
Anton
P.S. If you are having problems with Disqus slowing down your page, email me at anton@disqus.com and I will check it out.