Now that jQuery 2.x drops IE 8- support, a credible question is whether such libraries (which are more in the 95KB range, minified) are even necessary at all.
Quite outside of the time to download, there is a measurable cost to parse all of that boilerplate code as well. This has more of an impact on mobile, obviously.
Well my answer to the credible question is that jQuery has to normalize and fix bugs for just about all the browsers it supports, it's not just an oldIE problem. Plus, jQuery provides abstractions above the basic DOM operations which are pretty low level. So if you don't want to use jQuery you'd probably want some other library to provide equal or greater abstraction.
> Quite outside of the time to download, there is a measurable cost to parse all of that boilerplate code as well. This has more of an impact on mobile, obviously.
Measurable of course. Significant as a portion of the page load time for most pages? Well that's a different thing. And of course if you want it, jQuery 1.8+ provides the ability to create custom builds if you know you don't need parts of it.
As to the cost, assuming that everyone is pulling it from the same CDN (and there aren't the often considerable times simply to check an etag, which is the case with the overwhelming majority of jQuery hosts), an iPhone 4S takes about 80-100ms to simply parse the jQuery file (and given the plateauing of the integer core on on the An chips since, it is a reasonable assumption that every Apple variant since is similar). On each and every page render. A tenth of a second is a long time (and a lot of battery) for what is often of minimal value.