> It includes code to load up various analytics tools even if you never use them. For example, if I only use GA and Mixpanel, do I really want to serve the bytes for all the other plugins to my users? Maybe it would be useful to only package the JavaScript that a certain site needs.

It only has code that knows how to load each script, which isn't that much. So for example if you don't tell it to load chartbeat.js, it will never load chartbeat.js.

Regarding load time of pages, it's one thing we (Chartbeat) take really seriously, e.g. our default implementation waits until after onload before downloading anything and we keep our JS as light as possible. This script does, unfortunately, throw some of that out the window with it's implementation though. I'm working on my pull request already. :)