Screen Res and performance

[SEE]

The strange thing is that it runs better in widescreen at higher res than 4 x 3 in lower res.....

Swapping over to a 4 x3 monitor with lower res setting, I have a choice of resolutions in video options menu but FPS drops and 'Mirrors' no longer toggle but simply stop updating the view in them. In wide screen only one resolution is shown, better fps and the mirrors dissappear when toggled.