XCB

Bart Massey started the X C Binding at Portland State University in 2001, aimed squarely at Xlib's worst habit: lots of calls block until the server replies, and each wait costs a full network round trip. In XCB a request hands back a "cookie" you redeem only when you actually need the answer, so many requests can be in flight at once.

Nobody writes the library by hand. The core protocol and its extensions are described in XML (xcb-proto) and the C code is generated from that. Since the mid-2000s Xlib itself runs on top of XCB, which means almost every X program uses it without knowing. Direct users are mostly window managers such as i3 and awesome, where the saved milliseconds are noticeable.