Possivelmente uma questão relacionada: "É qualquer parte do software X.org multithreaded? "
Cite uma das respostas:
no livro de E. S. Raymond, o autor cita.
The X server, able to execute literally millions of ops/second, is not threaded; it uses a poll/select loop. Various efforts to make a multithreaded implementation have come to no good result. The costs of locking and unlocking get too high for something as performance-sensitive as graphics servers. -- Jim Gettys
Pelo que entendi, o lado do cliente pode ser multitheaded, só depende de implementações. E o mesmo se aplica ao wayland, os clientes podem ser multi-threaded, a partir de 01.org :
The client side library is designed to integrate well into toolkit event loops and work in both single-threaded and multi-threaded environments.