Making USER and GDI into Executive service components decreases their size and improves graphics performance. It eliminates shared memory buffers, reduces code size and complexity, and reduces thread transitions and context switches. For example, accessing GDI from the Win32 server on an Intel Pentium 90 takes at least 70 microseconds while accessing it in kernel mode takes 4 to 5 microseconds. When users are waiting for complex screen changes, these difference are noticeable. Graphics-dependent applications like Microsoft PowerPoint® run 15 to 20% faster on Windows NT 4.0.
Windows NT 4.0 is also much more efficient in its use of memory. It is estimated to save from 256K on machines running single applications to 1MB for users who have several applications running simultaneously. Some of this savings comes from simplifying the code needed to access graphics functions, but most comes from eliminating the 64K shared memory window used to optimize graphics performance in earlier versions of Windows NT.
These marked improvements allow us to incorporate new functionality, like the Windows 95 interface, with no net loss in performance or increase in memory requirements. Overall, the memory use and performance of Windows NT 4.0, should be very close to that of Windows NT 3.51.