Best practices are great but, at least in the case of performance, the question is more about the right practice. The only way to know is to:
- Measure
- Try
- Measure Again
- Compare/Contrast
It is almost always a trade-off with optimizing software. You have to consider how a change effects readability, scalability, security, deployment complexity, and probably a lot of other ity's I am not thinking of.
Having said all that the practices you are looking for are pretty much here.
To make my point though consider the rule Use a CDN. This seems like an obvious choice for something universal like jquery. You can use both MSFT and Google for this. You have the potential advantage of parallel download (depending up on the client browser) and the added possibility of already being in the client cache (from other site they may have visited). Plus they are likely serving it closer to the user than you are.
But the trade off to consider is that it is going to eat some bandwidth so if you are building something that is sitting in the Intranet, don't do this. You also introduce the possibility of failure from an external source. You can work around that by adding more code.
Perhaps most importantly though is that using a CDN is in direct contrast to the first rule, reduce HTTP requests. If you can combine and minimize all your js and load it more efficiently locally then you should do so. How do you know?
You have to:
- Measure
- Try
- Measure Again
- Compare/Contrast
Have fun!