Here's a better answer, straight from Microsoft - the entire document is interesting because it shows how sensitive they were both to adoption for NT vs. 95 but also how worried they were about other operating systems, particularly IBM's OS/2:
“Still, because Windows NT is a Windows-based operating system, in which all other environmental subsystems utilize the basic functions provided by Win32, and because the graphics and windowing subsystems have a very high rate of interaction with hardware (video drivers, mouse and keyboard drivers, and so forth), the Windows NT 4.0 design team decided to move that common functionality from user mode into kernel mode. In this way, the functions can be performed without shared memory buffers or paired threads, and with fewer thread and process context switches. This change as implemented in Windows NT 4.0 results in faster operation and reduced memory requirements, both visible benefits to the end user.”
…
“By making the Window Manager and GDI subsystems into Executive services, both operations decrease in size and improve in performance. This is largely due to the elimination of the shared memory buffers, as well as a substantial reduction in the code size and thread/process switching from the more complex client/server design. Consider, for example, the client/server transition time on a Pentium 100 which is in the order of 60-70 m seconds, whereas a kernel-mode transition is on the order of 4 to 5 m seconds. For screen graphics calls, this difference can become significant enough to be visible to the end user.”
…
“Moving Window Manager and GDI to kernel mode completely eliminates the buffer and related code complexities because all of the data in the client's address space is fully accessible from kernel mode. The total amount of working set memory saved with this change in the release of Windows NT 4.0 is estimated to be between 256K for machines that require few simultaneously open applications and 1 MB for users who consistently have many applications open at the same time.”
Don't miss this bit of optimism:
“Due to the modular design of Windows NT moving Window Manager and GDI to kernel mode will make no difference to the security subsystem or to the overall security of the operating system this will also have no effect on the C2 or E3 security certification evaluation, other than making it easier to document the internal architecture of Windows NT.”
https://msdn.microsoft.com/en-us/library/cc750820.aspx
“Still, because Windows NT is a Windows-based operating system, in which all other environmental subsystems utilize the basic functions provided by Win32, and because the graphics and windowing subsystems have a very high rate of interaction with hardware (video drivers, mouse and keyboard drivers, and so forth), the Windows NT 4.0 design team decided to move that common functionality from user mode into kernel mode. In this way, the functions can be performed without shared memory buffers or paired threads, and with fewer thread and process context switches. This change as implemented in Windows NT 4.0 results in faster operation and reduced memory requirements, both visible benefits to the end user.”
…
“By making the Window Manager and GDI subsystems into Executive services, both operations decrease in size and improve in performance. This is largely due to the elimination of the shared memory buffers, as well as a substantial reduction in the code size and thread/process switching from the more complex client/server design. Consider, for example, the client/server transition time on a Pentium 100 which is in the order of 60-70 m seconds, whereas a kernel-mode transition is on the order of 4 to 5 m seconds. For screen graphics calls, this difference can become significant enough to be visible to the end user.”
…
“Moving Window Manager and GDI to kernel mode completely eliminates the buffer and related code complexities because all of the data in the client's address space is fully accessible from kernel mode. The total amount of working set memory saved with this change in the release of Windows NT 4.0 is estimated to be between 256K for machines that require few simultaneously open applications and 1 MB for users who consistently have many applications open at the same time.”
Don't miss this bit of optimism:
“Due to the modular design of Windows NT moving Window Manager and GDI to kernel mode will make no difference to the security subsystem or to the overall security of the operating system this will also have no effect on the C2 or E3 security certification evaluation, other than making it easier to document the internal architecture of Windows NT.”