src/video/x11/SDL_x11video.c:X11_CreateWindow() contains the code below, which enables backing store if the X server supports it. Current Xorg implements backing store by redirecting the window storage to a separate pixmap using the composite layer. This prevents windows with backing store enabled from using page flipping for DRI2 buffer swaps. Instead, a buffer swap requires two copies (one from the DRI2 back buffer to the window pixmap, and one from the window pixmap to the screen pixmap), which might reduce performance significantly and causes visible tearing such as described in https://bugs.freedesktop.org/show_bug.cgi?id=79850 .

SDL2 doesn't seem to have this code anymore, so I think it would be be best to disable it by default, if not remove it altogether.

#if 0 /* This is an experiment - are the graphics faster now? - nope. */
	if ( SDL_getenv("SDL_VIDEO_X11_BACKINGSTORE") )
#endif
	/* Cache the window in the server, when possible */
	{
		Screen *xscreen;
		XSetWindowAttributes a;

		xscreen = ScreenOfDisplay(SDL_Display, SDL_Screen);
		a.backing_store = DoesBackingStore(xscreen);
		if ( a.backing_store != NotUseful ) {
			XChangeWindowAttributes(SDL_Display, SDL_Window,
			                        CWBackingStore, &a);
		}
	}