Nah, that is definitely not it. KR and SA are both built upon the GameByro engine which run such games as Civilization 4, The Elder Scrolls 4: Oblivion and Fallout 3. It can handle quite easily much more graphically intense games than Ultima Online, especially since the amount of polygons on screen are minimal. KR/SA use polygons only as 'billboards' to project the sprites onto, so maybe at most its using a couple hundred to a thousand polygons on screen for graphical display purposes.
No such luck.