So, don't put the js in the head. Nothing in HTML4.01 (I don't know about HTML5) bars inclusion of your script(s) (or links to your script(s)) in the body of the html.

This might be outdated information, but putting things in the body actually meant that many browsers had to request and load the javascript (why, it might contain document.write!) before continuing to parse the page -- hence, slowdown.

I'm very much unsure how browsers load a javascript in a head block, but any slowdown should only affect the first page load (when the scripts are cached), and the OP's concerns are best mitigated by setting up decent caching behaviour and not storing scripts on any external servers whose performance he cannot control.