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rendering is mostly a CPU task not a GPU one....

Correction, rendering is ENTIRELY a CPU task.
Most of what you see happening in your viewport is being done by your CPU too. The video card is using OpenGL to display lights and textures, transparencies, floor shadows, and shaders, and so on. 

Most, but not all of the display updating when you pose or move the camera is being handled by your processor. Your video card has to actually refresh all the scene elements in your display, but it's your processor that's handling the movement of the camera and the body parts. And it's not doing a very efficient job of it either, just like all 3D programs barely tap into the power of the GPU and OpenGL, and instead put most of the display burden on the processor.

If they wrote it in a way where the GPU was being used for everything display-wise, and also contributed to the rendering, Poser would be considerably faster than it is now.