Discovered when working on bug 3753. Modern QT applications are not using XCopyArea. Instead, they are using a backing store pixmap and redrawing everything every time. This means that we cannot use VNC CopyRect updates. From
“But hey, this means we have tons of XPutImage calls as part of the repainting, doesn’t it?” you are probably thinking. Luckily not… The raster graphics system is using the X Shared Memory extension to allocate the bits shared with an XPixmap so the end of a paint is a mere XCopyArea, which happens in-place on the X server, so it is quite cheap. Needless to say this is a solution that works best on local displays, but that does cover most of the Linux desktops out there, so it should be a gain in general."
Although it's possible to change the graphics system, it is unclear if you can get the old behaviour.