Video memory bottleneck

Writing into video memory seems to be very slow. Which means that the only way to have a fast FPS is not to write into Video memory in real time. That restricts you to the video card''s capabilities. Even if you had a pentagon''s super computer, which is capable of doing Raytraced3D in real time, you still be restricted to either the video card''s capabilities or to the slow speed of writing into video memory. So instead of investing your money on a better cpu, you comprise on your cpu so you can buy another processor that can do only certain tasks(3D Cards). Does the DDR memory based cards do not suffer from a very slow video memory direct writting?