The image for the post is a visual/artistic experiment playing with simultanous contrast resulting from other experiments these days. An over-saturated colored grid overlayed on a grayscale image causes the grayscale cells to be perceived as having color.
The processing needed to create the above image happened along with unrelated but significant code improvements In the last couple of weeks. I have been visiting mitch - a prolific GIMP contributors for collaboration - and lots of progress has been - and is still - being made on babl, GEGL and GIMP.
Through profiling and effective discussions of why things are not behaving like our code and we expect, the GTK3 / master branch of GIMP is now snappy and thus usable again - a development snapshot is probably coming soon. Part of the problem was that with GTK3, any changes to the status bar caused full window relayout and redraw, another problem was that uninitialized memory in one babl code path caused GIMPs canvas redraw to be really slow. These things being slow; slowed down everything painting, zooming, menu browsing and more. A release of babl (tarballs now made with meson instead of autotools) is out with the latter fix which also is good for GIMP 2.10 where full-canvas-redraw problem is not present - but color management with matrix profiles will again be as fast, or faster than it was a year ago.