> Calls that don't require a response from the X server are queued in a buffer to be sent as a batch of requests to the server. Those that require a response flush all the buffered requests and then block until the response is received. [1]

X11, the protocol is a distributed systems protocol, built on asynchronous message passing, that outputs to the screen as a side effect. Xlib hides the nature of the underlying protocol, and makes it very hard to pipeline things that should be pipelined for maximum performance. Xcb separates out sending a request and waiting for a response, so if you absolutely need the response before you continue, you can do that, but you can also request many things and then wait when you need them.

[1] https://www.x.org/wiki/guide/xlib-and-xcb/