How does this script impact the performance of a site?

Specifically...

1. 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.

2. The script still blocks the onload event. See this post for a way around that: http://calendar.perfplanet.com/2012/the-non-blocking-script-...

3. How does having multiple analytics solutions on a page affect the load time of that page?

Lastly, I couldn't tell from the docs how to call methods before the script has finished loading. Is it similar to GA's method queue pattern?