I've often have choppy gameplay when playing the game on High/Ultra settings (especially in towns). I have a medium end computer with a 550Ti and I recently found a solution to this problem that I'd like to post for other players with the same issue.
There are hidden settings that improve quality or performance that are not visible within the game settings. I discovered this when I was experimenting with the presets. If you have a low to medium end machine like me and you want to play the game with all visible settings on high or ultra without choppiness, then do these steps:
1) Set the preset setting to "Minimum" or "Low".
2) Set all settings including the textures to "High".
3) Turn on all settings on the bottom.
By setting the preset to "Minimum" you are turning off some graphic processing effects not seen in the menu. The difference can be seen best when looking out over the water. However, you will now be able to play the game using "High" textures with ambient occlusion turned on.
Keep in mind that your preset will now be displayed as "Custom". So it isn't exactly "High" or "Ultra" but at the very least you can play it with the higher textures.