X11 uses a network extensible protocol that allows cross-machine and even cross-vendor and cross-operating system interoperability of X clients and servers. X11's network extensibility is in high contrast to the lack of network extensibility in Microsoft Windows. Most X clients run reasonably well locally and remotely. In most cases, the user is unaware when a client is running remotely. The success of X's transparent network extensibility has even created an entirely new network device: the X terminal [8].
The X community often lauds network extensibility over PC users of Microsoft Windows, but Microsoft Windows has gained widespread usage despite the lack of network extensibility.
For most PC users, contemporary PC technology provides plenty of power for tasks such as word processing, spreadsheets, and electronic mail. The idea of remote execution of windowed applications is considered interesting but frivolous. And in the homogeneous world of PCs, support for heterogeneous machines and operating systems is largely unnecessary.
For Microsoft Windows users, the network can be an important source of services (file systems, databases, and electronic mail) but the graphical front ends to such services can easily be implemented as a local Microsoft Windows program. Even when the storage or computational burden of an application like a database is beyond the capability of a desktop machine, the user's graphical application need be only an interface between the user and another computer managing the database. If services are sufficiently network extensible, a network extensible window system may not be necessary.
One should be careful not to draw the wrong conclusion. The point is not that network extensible window systems are unimportant, but that in a homogeneous environment using desktop computers with more than adequate power for common applications, network extensible window systems are not necessary. In a heterogeneous environment with users sharing computer resources beyond the capability of the machine on their desk, network extensible window systems have proven to be vitally important.
The important point is X11 is competing with window systems that are not network extensible but that derive important performance benefits from the optimization opportunities available to a purely local window system implementation.