with any implementation of X, he can connect his server to the client software in Alice's computer over a network, and be working on completely separate things, sharing the resources of one computer
Although to be fair; this design choice has taken a performance toll on every other use of X, to the detriment of almost every desktop unix user the last 20 years.
...but only if Alice is indeed a team player who wants to share her precious computer resources. Linux would also be a solution but nobody wants to hear about that.