Remote Scripts - best practice for speed

Is it better to link to a remote script, or embed it directly on your site?

[MadDogMike]

So, I use a script like this on my website:

<script type="text/javascript" src="http://scriptsource.com/whatever-min.js"></script>

From a performance/speed perspective, is it better to pull a script from a remote source like that, or embed the whole script directly on my site?

[phranque]

it's a tradeoff.
if you put a script in your document it saves an extra request for a server resource.
however an external script can be cached by the browser and reused for later visits or on other pages.
an external script also reduces the size of the html document.
in some cases, serving an external script from a separate domain, especially if it's on a content delivery network, can help performance.
if the script is served from a cookieless domain that can also improve performance.

[lucy24]

in some cases, serving an external script from a separate domain, especially if it's on a content delivery network, can help performance.

Can the connection be so much faster that it outweighs the extra time involved in looking up a whole other domain?

[DrDoc]

No, first connections anywhere are always made at a loss in performance. But, assuming you are not using some fringe library, chances are that people have common domains cached in their browser/router/DNS.

[phranque]

your browser limits the number of simultaneous connections to a server, so the extra DNS lookup is yet another tradeoff vs the total number of connections required (aka "in some cases")

the CDN tradeoff is more DNS lookups vs lower latency and perhaps more cached resources.