we had redraw loops since a long time - lately our drawing speed regressed additionally

reactOS r74600
Repainting without themes in classic mode got a serious hit-back recently.
When I select another listview item in rapps the selection change is heavily delayed while the scroll-bar arrows in the details view are heavily flashing.
Furthermore when I open up some apps-windows, their content does not really seem to be drawn "as a whole", but you can see the individual parts of the window drawn one after each other. video.webm
Disabling + stopping themesvc + restart does not help.
But when I enable some theme e.g: Lautus the effect seems to be gone or is greatly reduced.

0.4.5RC2 was still much better. It flashed too, but the drawing was somehow much faster then. The regression is very fresh. Most notable draw-speed regressed somewhere between r74479-r74500.