Although KDE-Next will be happy to run on top of either XCB or Xlib, (or OSX or MS-Windows too, our highest-performance platform will consist of the Window Manager being Window Manager, Compositor, and Display Server in a single process. It will be able to translate X11 drawing primitives with abut the same efficiency as running X11 natively (i.e., almost no additional overhead), but the big payoff appears when the Client Applications are themselves running as Wayland- aware programs.
Qt Version 5, which will be out in just a few months, provides runtime "plugins" for the platform-level interfaces. To some extent, KWin (the Window manager/compositor) will take advantage of Wayland auto-magically. But, in order to achieve the highest possible performance, we will be doing some Wayland specific coding in KWin -- making it Wayland-aware when Wayland is available, and "downgrading" into the alternative XCB or XLIB code paths when we have to.
Modern video cards are smart, the X11 protocol (even with extensions such as "COMPOSITE") doesn't take full advantage of their capabilities.
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