Before playback, are you first Rendering the Image/Clip/Title (hitting Enter/Return)?
When Keyframes are placed, they are Linear. One can change the Interpolation. However, for a long Pan, the Linear Keyframe would only affect the very beginning, and then possibly the end. The "tweens," should be smooth, once Rendered.
Looked for the limit on Still Images for PrPro CS 6, but could not find it. There was a change, from CS 4 to CS 5, and I think that has remained the same, through CS 6. I know that I have it somewhere, but finding the "WHERE" is proving tougher, than I thought.
Now, even with the MPE/CUDA support, as of CS 5, handling large Still Images can pose resource issues with one's machine. One thing that you could do is Crop that Image to just what you need on the width (so long as the total pixels do not exceed PrPro's capabilities), and the height, to just what you need, say 1080. That removes many unused pixels, unless you are also animating "Tilts" on the height too.
Note that AE can handle MUCH larger Still Images, than can PrPro, so that might be an alternative, if you have access to it.