Self-hosting is no longer the performance drain it used to be now that HTTP/2 is a thing, as soon as the browser parses the page, the JS file is only 1 RTT away. Using a CDN for a single asset like jQuery requires a DNS lookup, TCP connection and TLS negotiation - that's 3 RTTs plus unknown latency from the user's resolver before the HTTP request is even sent. The performance difference in the single request scenario is likely negligible and the benefits of not relying on a 3rd party can outweigh the latency savings.
