I don't think it gives a performance boost to host the files yourself. Its possible on a good server that they might load faster from your site than google, but in many cases that will not be true. Using a cdn takes some load off of your server since it does not serve those files therefore it has additional capacity for other requests. Also, the js/css gets cached by the browser so if the user has already visited any site that uses these files from the google CDN then the user likely already has it in cache so performance is even better. Whether self hosted or not these files get cached and are typically only loaded on the first visited page. If serving from yuor own site it will always load the js/css the first page and should generally cache it after that, but using the CDN you get the same benefit on a first visit to your site if the user has visited another site using the google CDN for these files.
You may percieve a perfomance boost running mojo from localhost because local files will serve faster on the machine than files coming from the internet. On production internet sites I don't think you'll notice generally but its a good practice when possible to use the CDN.
Best,
Joe