When Windows NT was first designed, the Win32 environment subsystem was designed as a peer to the environment subsystems supporting applications in MS-DOS, POSIX, and OS/2. However, applications and other subsystems needed to use the graphics, windowing, and messaging functions in the Win32 subsystem. To avoid duplicating these functions, the Win32 subsystem was used as a server for graphics functions to all subsystems.
This design worked respectably for Windows NT 3.5 and 3.51, but it underestimated the volume and frequency of graphics calls. Having functions as basic as messaging and window control in a separate process generated substantial memory overhead from client/server message passing, data gathering, and managing multiple threads. It also required multiple context switches, which consume CPU cycles as well as memory. The volume of graphics support calls per second degraded the performance of the system. It was clear that a redesign of this facet in Windows NT 4.0 could reclaim these wasted system resources and improve performance.