Same thing happens to me...
My theory is that this is caused by game rendering alpha channel of the .png images.
So, ironsights image with no transparency of pixels other than 100% or 0% should improve the performance.
I can offer you such image, just place it in your game's png folder and replace the "semi.png" image with it.
http://www.mediafire.com/download.php?1ji2dl414i5mq59Didn't seem to do wonders for me, but i got improvement of about 3 fps, and thats good,
considering that im playing this game on a toaster ^^
If this doesnt do anything, then the culprit must be the rendering of the view in this mode or a memory leak,
there is no fix for this unless the dev makes a patch...