please attach the output of
   qdbus org.kde.KWin /KWin supportInformation

100% cpu usage implies a lifelock across the eventloop - can you callgrind kwin_x11?
Also please check whether this is related to compositing, SHIFT+Alt+F12 toggles the compositor.

Is there some information non how to callgrind kwin?

Disabling Compositing normalized the cpu usage.

valgrind --tool=callgrind kwin_x11 --replace &

Since valgrind severely slows down execution, you should be comfortable with controlling the system from VT1 or ssh (your GUI login will become virtually unusable)

Let's maybe try to narrow the issue first ;-)

- Please attach forementioned supportInformation and (assuming this is GL compositing)
- then run "kcmshell5 kwincompositing" (config dialog, the compositor can be suspended at this time) and
- set the backend to xrender compositing, (in doubt resume the compositor and) see if the issue remains.
- If not, switch back to OpenGL (assuming you're using it)
- but set the tearing prevention to "none"

Find attached the desired information. Xrender is fast, OpenGL is slow, it doesn't matter which version and whether tearing prevention is switched on or off.

Version
=======
KWin version: 5.4.0
Qt Version: 5.5.0
Qt compile version: 5.5.0
XCB compile version: 1.11

Operation Mode: X11 only

Build Options
=============
KWIN_BUILD_DECORATIONS: yes
KWIN_BUILD_TABBOX: yes
KWIN_BUILD_ACTIVITIES: yes
HAVE_WAYLAND: yes
HAVE_WAYLAND_EGL: yes
HAVE_WAYLAND_CURSOR: yes
HAVE_XKB: yes
HAVE_INPUT: yes
HAVE_DRM: yes
HAVE_GBM: yes
HAVE_X11_XCB: yes

X11
===
Vendor: The X.Org Foundation
Vendor Release: 11702000                                                                                                                                                                             
Protocol Version/Revision: 11/0                                                                                                                                                                      
SHAPE: yes; Version: 0x11                                                                                                                                                                            
RANDR: yes; Version: 0x14                                                                                                                                                                            
DAMAGE: yes; Version: 0x11                                                                                                                                                                           
Composite: yes; Version: 0x4                                                                                                                                                                         
RENDER: yes; Version: 0xb                                                                                                                                                                            
XFIXES: yes; Version: 0x50                                                                                                                                                                           
SYNC: yes; Version: 0x31                                                                                                                                                                             
GLX: yes; Version: 0x0                                                                                                                                                                               
                                                                                                                                                                                                     
Decoration
==========
Plugin: org.kde.breeze
Theme: 
Blur: 0
onAllDesktopsAvailable: false
alphaChannelSupported: true
closeOnDoubleClickOnMenu: false
decorationButtonsLeft: 0, 2
decorationButtonsRight: 6, 3, 4, 5
borderSize: 3
gridUnit: 10
font: Oxygen-Sans,10,-1,5,50,0,0,0,0,0
smallSpacing: 2
largeSpacing: 10

Options
=======
focusPolicy: 0
nextFocusPrefersMouse: false
clickRaise: true
autoRaise: false
autoRaiseInterval: 0
delayFocusInterval: 0
shadeHover: false
shadeHoverInterval: 250
separateScreenFocus: false
placement: 4
focusPolicyIsReasonable: true
borderSnapZone: 10
windowSnapZone: 10
centerSnapZone: 0
snapOnlyWhenOverlapping: false
rollOverDesktops: true
focusStealingPreventionLevel: 1
legacyFullscreenSupport: false
operationTitlebarDblClick: 5000
operationMaxButtonLeftClick: 5000
operationMaxButtonMiddleClick: 5015
operationMaxButtonRightClick: 5014
commandActiveTitlebar1: 0
commandActiveTitlebar2: 30
commandActiveTitlebar3: 2
commandInactiveTitlebar1: 4
commandInactiveTitlebar2: 30
commandInactiveTitlebar3: 2
commandWindow1: 7
commandWindow2: 8
commandWindow3: 8
commandWindowWheel: 31
commandAll1: 10
commandAll2: 3
commandAll3: 14
keyCmdAllModKey: 16777251
showGeometryTip: false
condensedTitle: false
electricBorderMaximize: true
electricBorderTiling: true
electricBorderCornerRatio: 0.25
borderlessMaximizedWindows: false
killPingTimeout: 5000
hideUtilityWindowsForInactive: true
inactiveTabsSkipTaskbar: false
autogroupSimilarWindows: false
autogroupInForeground: true
compositingMode: 1
useCompositing: true
compositingInitialized: true
hiddenPreviews: 1
unredirectFullscreen: false
glSmoothScale: 1
colorCorrected: false
xrenderSmoothScale: false
maxFpsInterval: 16666666
refreshRate: 0
vBlankTime: 6000000
glStrictBinding: true
glStrictBindingFollowsDriver: true
glCoreProfile: true
glPreferBufferSwap: 101
glPlatformInterface: 1

Screen Edges
============
desktopSwitching: false
desktopSwitchingMovingClients: false
cursorPushBackDistance: 1x1
timeThreshold: 150
reActivateThreshold: 350
actionTopLeft: 0
actionTop: 0
actionTopRight: 0
actionRight: 0
actionBottomRight: 0
actionBottom: 0
actionBottomLeft: 0
actionLeft: 0

Screens
=======
Multi-Head: no
Active screen follows mouse:  no
Number of Screens: 1

Screen 0:
---------
Name: LVDS1
Geometry: 0,0,1366x768
Refresh Rate: 60.0204


Compositing
===========
Compositing is active
Compositing Type: OpenGL
OpenGL vendor string: Intel Open Source Technology Center
OpenGL renderer string: Mesa DRI Intel(R) Pineview M 
OpenGL version string: 2.1 Mesa 10.6.5
OpenGL platform interface: GLX
OpenGL shading language version string: 1.20
Driver: Intel
GPU class: Unknown
OpenGL version: 2.1
GLSL version: 1.20
Mesa version: 10.6.5
X server version: 1.17.2
Linux kernel version: 4.1.6
Direct rendering: Requires strict binding: yes
GLSL shaders:  yes
Texture NPOT support:  yes
Virtual Machine:  no
OpenGL 2 Shaders are used
Painting blocks for vertical retrace:  no

Loaded Effects:
---------------
zoom
slidingpopups
kwin4_effect_login
slide
screenshot
kwin4_effect_windowaperture
minimizeanimation
kwin4_effect_translucency
desktopgrid
kwin4_effect_maximize
kwin4_effect_fade
presentwindows
highlightwindow
kwin4_effect_dialogparent
blur
contrast
logout
dashboard
startupfeedback
screenedge
kscreen

Currently Active Effects:
-------------------------
blur
contrast

Effect Settings:
----------------
zoom:
zoomFactor: 1.2
mousePointer: 0
mouseTracking: 0
enableFocusTracking: false
followFocus: true
focusDelay: 350
moveFactor: 20
targetZoom: 1

slidingpopups:
fadeInTime: 150
fadeOutTime: 250

kwin4_effect_login:

slide:

screenshot:

kwin4_effect_windowaperture:

minimizeanimation:

kwin4_effect_translucency:

desktopgrid:
zoomDuration: 300
border: 10
desktopNameAlignment: 0
layoutMode: 0
customLayoutRows: 2
usePresentWindows: true

kwin4_effect_maximize:

kwin4_effect_fade:

presentwindows:
layoutMode: 0
showCaptions: true
showIcons: true
doNotCloseWindows: false
ignoreMinimized: false
accuracy: 20
fillGaps: true
fadeDuration: 150
showPanel: false
leftButtonWindow: 1
rightButtonWindow: 2
middleButtonWindow: 0
leftButtonDesktop: 2
middleButtonDesktop: 0
rightButtonDesktop: 0

highlightwindow:

kwin4_effect_dialogparent:

blur:
blurRadius: 12
cacheTexture: true

contrast:

logout:
useBlur: true

dashboard:
brightness: 0.5
saturation: 0.5
blur: false

startupfeedback:
type: 1

screenedge:

kscreen:

Try
- the EGL backend and/or to
- disable blur and contrast effects

On large invocation, latter would significantly stress the IGP (and may fall over to CPU emulation), though I would not expect 100% CPU load here.

Another wild guess on blaming the driver, add
   /etc/X11/xorg.conf.d/20-intel.conf
containing:

Section "Device"
   Identifier  "Intel Graphics"
   Driver      "intel"
   Option      "AccelMethod"  "uxa"
   Option      "TearFree"    "false"
EndSection


Then restart the X11 server (log out and in, in doubt reboot)

It was the blur effect. Disabling this one resulted in a snappy desktop again. Did you change anything from 5.2->5.3/4?

We have one or two bug fixes, but nothing which would cause that.

I rather think it's a change in the driver to enable Shaders for the hardware.

Anyway. Thanks for your kind an instant help. Outstanding. And maybe this bug report helps you somehow. If you need further testing, let me know!

Try to lower the blur strength.
It's internally capped but the driver may have started to announce larger values reg. the program variable limits.

Also in older (than 5.2 must have been during 5.0 or 5.1) versions there was a bug that interpreted the configured strength too low.

Set the value (in the config dialog) to 2 for a start (resulting in a gaussian kernel of 4, presently is 12 and that should, if not capped, be too much for the HW, resulting in CPU emulation)

This bug is now quite old with no particularly useful information. 

Please do reopen if it's still an issue with feedback relating to the comments above