I found that scale of window by X (800x600 -> 1800x600) don't slow down the rendering.scale of window by Y (800x600 -> 800x1200) DOES slow down (600->1200, slower by 2 times).so the reason is: Canvas.TextOut (or LCLIntf.ExtTextOut) is slowing down all render.I also see that debugger stops inside the ExtTextOut (most of the time, in 80%).I cannot solve it, TextOut (ExtTextOut) on linux is not known code to me.the same slowdown occurs on gtk2 and qt5.so it is mostly Lazarus issue of gtk2/qt5 codes.