A very large number of them were code refactors to take better advantage of gzip; Richard Gibson (gibson042) has become a master of that [1]. It turns out that a good style guide can actually help improve gzip compression as well [2]. Others were things like an optimization for $("body") which we pulled out [3]. Its main job seems to have been to look good on a circa-2008 benchmark that didn't cache its selectors. The Sizzle optimizations that Timmy Willison (timmywil) did pretty much made the jQuery-side optimizations unnecessary anyway.

[1] https://github.com/jquery/jquery/pull/788 [2] https://github.com/jquery/jquery/pull/840 [3] https://github.com/jquery/jquery/commit/9ebc27db54194c88fa7a...