We do very little processing and so the main time will be just the time it takes to write large files. In fact images are stored in PNG format, but the time overhead for that should be negligable.

There is one way to make a very subtantial imporvement and that's to encode the images as JPEG. I know for quality reasons you may not like the idea, but Xara does not re-encode JPGs, so Xara does a lossless save. (i.e. there will be some small loss in quality encoding into JPG the first time, but at least Xara does not make it any worse). When saving files Xara just saves the original JPG image, and since they are typically 10x small than raw images, that means a 5 to 10 times speed up saving files.

(At least be thankful you're not using AI - that takes minutes, literally, every timeyou save a file with even modest bitmaps in - last time I timed it we were more than 10 times faster than Illustrator)