The only slowdowns I have experienced in a sandbox is with IE7, which is very slow to load, and gets very sluggish in responding to such things as opening new tabs. However, I use AM browser (an IE shell) where I usually have from 15-30 tab open, as well as TheWorld browser, Firefox, Safari and Opera and have not noticed any major slowdowns in any of those. I have not tried IE7 no-addons or gone through the ritual of disabling add-ons one at a time to try and pinpoint the problem (if that is the problem). I am currently using sandboxie 2.94.23 (beta).
One of the things I like most about Sandboxie is if you post issues on the forum, tzuk or someone else will reply in very short order, and tzuk sometimes is able to fix a problem in minutes. I've done a lot of beta testing with him and he is extremely responsive. The only reason I've found to surf outside of the sandbox is when I want to do MS Updates manually. And in case you haven't tried any recent versions, Sandboxie now has a Quick Recovery function, so when you download a program or file in the sandbox, you get a dialog immediately upon completion of the download asking if you want to save the file to the hard drive location where you saved it. This feature is not enabled by default (for security reasons), but it only takes a second to enable in the SB Control Window.
What? Me Worry? I surf in a sandbox!!