Not sure I agree with the non-use of libraries like jQuery from a "common source" since most of the time they will be cached by browsers from previous requests, I can say multiple .js files on a site are not in any way ideal.
When a "script" is encountered by a browser it will stop all concurrent downloads from the host until the download of the script is completely downloaded, so it can affect the overall page load time negatively for sure.
If you need to include JS/jQuery, do it all in one file right before the </body> tag of a page... I use jQuery from a "standard source" and unless I empty my cache it's a non-slowdown... If I do, it takes a second or two extra, but if the HTML is displayed first I hardly even notice the extra time to "grab" the jQuery, because browsers will open another connection to a different host name.