Just to say that it's the other way around. A synchronous protocol requires responses or acknowledgment to come back before sending new requests, while an asynchronous one doesn't. As such, asynchronous protocols are the ones that lag less, because if you send an intensive request and then a less intensive one, you can still receive the second answer rapidly.
Originally Posted by nomadewolf
That being said, Wayland is actually asynchronous while X11 isn't much, so this is indeed an improvement brought by Wayland's design.