my site is so slow. what could it be?

I have a multisite install with two themes in the same domain. One primary and one subdirectory.
My site seems so slow when I'm working on it and when I visit it. I tried wp super cache and it caused issues with my sitemap. I also didn't notice any difference with it installed, despite numerous tweaks and forum research. I'm about to try w3 total cache but I'm reluctant. When I uninstalled wp super cache it broke one of my sites and I had to rebuild htaccess. What could be slowing my site down so much, and how can I fix it? It's http://mybizpower.com