I believe this is due to the draw method, the origins of all this "stream of data", and drawing a screen, were to draw it line by line in front of the viewer very fast. (crt)
The data is still being transfered between things in much the same linear data stream as it was before.
_______________________________________line1 _______________________________________line2 (etc) It is just being displayed now in one single refresh of the screen. When you rotate the thing, everything changes:
_ <-- that goes | there and _ this goes | here and _ on & on | Till all the data is streamed lineraly to the display device, completly rearranged. It is much different work, vrses the way the stuff was originally designed.
This is a pretty lame explaination, but it might explain it quick enough.
If both pieces of hardware were designed to work in the different aspect of the "rotated" display (not actually rotated), and the data stream didnt have to be all re-arraged, then there is no reason why a "portrait" display would be any slower. It just isnt done that way. It is very possible that a monitor exists that just has a different aspect, where it is taller vrses wide.
If this process of rotating is done a lot better optomised, works with the hardware better, it should be able to be done without destroying everything.
Another thing that can happen is sub-pixel rendering (clear type) doesnt work right, because of how the 3 cells of color are arranged horizontal, and now are rotated. So done proper that all has to be changed too. From what I understand so far it isnt.
Speaking of Renders, for the things that act different, and your getting a failed display of a video player, change the render type. The "overlay" render type only cuts out a hole in the software, then this hole is filled by the hardware. Most all the video players including microsofts have a setting somewhere to change the render type.