https://bugs.kde.org/show_bug.cgi?id=523971

--- Comment #4 from Noah Davis <[email protected]> ---
It seems the delay is inconsistent and depends heavily on how hard the CPU has
to work, which makes sense since the image data has to be sent and processed
into QImage objects on the CPU.

With a 1080p laptop screen, a 4K monitor connected via HDMI and a 4K monitor
connected via DisplayLink dock, I normally have something near these times for
receiving screenshot request replies for a rectangle screenshot.

>15:54:00.431 spectacle: unknown QDBusMessage(type=MethodCall, 
>service=org.kde.KWin.ScreenShot2, path=/org/kde/KWin/ScreenShot2, 
>interface=org.kde.KWin.ScreenShot2, member=CaptureScreen, signature=, 
>contents=("eDP-1", [QVariantMap], [Unix FD: valid]) )
>15:54:00.431 spectacle: unknown QDBusMessage(type=MethodCall, 
>service=org.kde.KWin.ScreenShot2, path=/org/kde/KWin/ScreenShot2, 
>interface=org.kde.KWin.ScreenShot2, member=CaptureScreen, signature=, 
>contents=("HDMI-A-1", [QVariantMap], [Unix FD: valid]) )
>15:54:00.431 spectacle: unknown QDBusMessage(type=MethodCall, 
>service=org.kde.KWin.ScreenShot2, path=/org/kde/KWin/ScreenShot2, 
>interface=org.kde.KWin.ScreenShot2, member=CaptureScreen, signature=, 
>contents=("DVI-I-1", [QVariantMap], [Unix FD: valid]) )
>15:54:00.453 spectacle: unknown Time to receive CaptureScreen reply: 22.3493ms
>15:54:00.490 spectacle: unknown Time to receive CaptureScreen reply: 59.324ms
>15:54:00.499 spectacle: unknown Time to receive CaptureScreen reply: 67.909ms
>15:54:00.519 spectacle: unknown Time to receive all replies and read image 
>data: 88.132ms

For screenshot capture latency, the time to read image data is not so
important, mainly just the time to receive a reply.

If I start something CPU heavy like building the latest version of dev branch
qtbase, I get these times:

>15:58:26.455 spectacle: unknown QDBusMessage(type=MethodCall, 
>service=org.kde.KWin.ScreenShot2, path=/org/kde/KWin/ScreenShot2, 
>interface=org.kde.KWin.ScreenShot2, member=CaptureScreen, signature=, 
>contents=("eDP-1", [QVariantMap], [Unix FD: valid]) )
>15:58:26.455 spectacle: unknown QDBusMessage(type=MethodCall, 
>service=org.kde.KWin.ScreenShot2, path=/org/kde/KWin/ScreenShot2, 
>interface=org.kde.KWin.ScreenShot2, member=CaptureScreen, signature=, 
>contents=("HDMI-A-1", [QVariantMap], [Unix FD: valid]) )
>15:58:26.455 spectacle: unknown QDBusMessage(type=MethodCall, 
>service=org.kde.KWin.ScreenShot2, path=/org/kde/KWin/ScreenShot2, 
>interface=org.kde.KWin.ScreenShot2, member=CaptureScreen, signature=, 
>contents=("DVI-I-1", [QVariantMap], [Unix FD: valid]) )
>15:58:26.519 spectacle: unknown Time to receive CaptureScreen reply: 64.3373ms
>15:58:26.715 spectacle: unknown Time to receive CaptureScreen reply: 260.093ms
>15:58:26.765 spectacle: unknown Time to receive CaptureScreen reply: 309.948ms
>15:58:26.849 spectacle: unknown Time to receive all replies and read image 
>data: 393.89ms

Basically 2-5x slower.

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