Hi Jon,
thank you for picking up on this.
If you are changing the size of the active display only a couple of times then it is not a problem
But if you are changing it say 10 or more times then the thread count goes up and the program slows down greatly
My workaround is to to keep the size of the active display fixed - and then when I display a new region I change the Y aspect ratio of the display to get the correct bounding box
(i am only working in 2D). Then when the images are viewed on the web-site we stretch the images by this Y aspect ratio
...Henry