Since upgrading to 5.12.x on neon LTS, my system experience a memory leak (RAM keep building up until it eats all of the RAM). I have another machine, both intel graphics, and similar desktop config (same wallpapers, widgets, etc - and no using slideshow). The fix proposed on bug 368838 391063 which consist of QSG_RENDER_LOOP=threaded works for me but thought it should be implemented by default? I tried pkill plasmashell; QSG_RENDER_LOOP=threaded kstart plasmashell and since no more leak it seems like.
Please reopen if you have issues with Qt 5.11 or newer. *** This bug has been marked as a duplicate of bug 368838 ***