[penagwin]

I'm not taking a side, just trying to add some numbers. Let's ignore the privacy/uptime concerns for the sake of this comment.

If every site you visit has 350kb of stuff that would benefit from a CDN JS but also some CSS and fonts (google fonts, bootstrap, etc.) If you visit 50 pages a day in a 30 day month, that's a little over 500mb of data.

.35mb x 50sites x 30days = 525mb

That would be a ton of easily avoidable data in regards to mobile plans depending on where you are. This number isn't 100% accurate though, many "normal" (read - not techy hackernews readers) might only visit say a dozen sites a day or less (let's ignore apps like facebook/snapchat/etc). Even that might be a stretch.

Then again students and other "savy" users might be going across hundreds of new sites a day.

For you the host? Unless you're a massive beast, most of us "hobbiests" fit within the free bandwidth of 5$ vps services anyway.

[robin_reala]

That’s assuming every site is using the same CDN and the same version of the library. Seeing as that’s not the case you can cut that by at least an order of magnitude. Secondly, most users visiting 50 pages a day will not visit 50 sites a day but more like 5 pages a site across 10 sites, or even 10 pages a site on 5 sites. Now let’s add in to the user the external privacy cost of being tracked across multiple sites and it starts to look a little less appetising again from the user’s point of view.

[manigandham]

Unfortunately everyone still ends up using many different versions or adding other unnecessary querystring parameters so that each site still effectively ends up with their own file to download.