Reading a photo image with the window format is terribly slow, if the Aero design is enabled.
Reading the canvas included in test-suite visualtests takes 4.4 seconds with Aero enabled; 0.2 seconds with Aero disabled.
The GetPixel function seems to be the cause.
I doubt if Aero is the only reason for this bug. I ran into this bug also when I tested a script using Img -format window running under WINE (V 1.4.2). I switched all WINE Libs to Win98, but still the common symptoms (CPU load 100%, taking long time to finish, capturing only fraction of the image) were present. This is a real serious bug, since in Windows Version from 8.0 upwards Aero can't be disabled anymore. Actually there is no proper possibility to write a canvas to an image file under these conditions, rendering applications making use of this useless.
Fixed with Img 1.4.5