"World of Warcraft" expansions do not make the game run more slowly on an older computer. While any increase in graphical quality increases the required system specs, Blizzard's programming ensures the effects are minimized.
With the 2014 expansion "Warlords of Draenor," "World of Warcraft's" player models boast significantly more moving parts in their animations. Although this means the new models require more memory from the player's computer than before, Blizzard developers have already created new technology to compensate. Some of these changes were already added to the game's systems prior to the expansion even being released. Blizzard continuously optimizes new content for players throughout an expansion's duration.