While it is true that HTTPS does not come for free in terms of performance, if a single-core PC can do on the order of 100-150 RSA-2048 operations per second, I don't see client-side performance being greatly impacted by loading one specific file over HTTPS. Caching could potentially be an issue, but Google is already serving the jQuery Javascript source code file with a Cache-Control: public response header, which tells a conformant client that the response may optionally be cached. Also, not only is the URL itself versioned (so it is determinable without making a HTTP request whether the file exists in the cache or not), the cache-control directive also includes a max-age specifier and there is an Age header. All this means that browsers have all the information they need to do proper, efficient caching even before establishing the TCP connection to, let alone the TLS session with, Google's servers, once the file has been downloaded even just once.