SUMMARY

Kwin_wayland uses a whole CPU, spinning at 100%. Making the system "lagging" from the session start.
This does not reproduce on my laptop with similar software setup.
The concerned system uses GPU AMD Radeon RX 560 with amdgpu driver.
I can give more details of course.

The gdb trace was obtained by ssh'ing to the system and sudo gdb -pid `pidof kwin_wayland`

Thread 1 "kwin_wayland" received signal SIGINT, Interrupt.
0x00007fc9e60cec63 in ?? () from /usr/lib/x86_64-linux-gnu/libQt5Gui.so.5
(gdb) bt
#0  0x00007fc9e60cec63 in  () at /usr/lib/x86_64-linux-gnu/libQt5Gui.so.5
#1  0x00007fc9e5f97219 in  () at /usr/lib/x86_64-linux-gnu/libQt5Gui.so.5
#2  0x00007fc9e5fb0cf5 in QPainter::drawImage(QRectF const&, QImage const&, QRectF const&, QFlags<Qt::ImageConversionFlag>) () at /usr/lib/x86_64-linux-gnu/libQt5Gui.so.5
#3  0x00007fc9dd8e50c7 in KWin::SceneQPainter::Window::renderShadow(QPainter*) (painter=0x55c403f7f520, this=<optimized out>) at /usr/include/x86_64-linux-gnu/qt5/QtCore/qflags.h:120
#4  0x00007fc9dd8e50c7 in KWin::SceneQPainter::Window::renderShadow(QPainter*) (this=<optimized out>, painter=0x55c403f7f520) at /home/meven/kde/src/kwin/plugins/scenes/qpainter/scene_qpainter.cpp:322
#5  0x00007fc9dd8e876e in KWin::SceneQPainter::Window::performPaint(int, QRegion, KWin::WindowPaintData) (this=this@entry=0x55c4046742a0, mask=mask@entry=10, region=..., data=...) at /home/meven/kde/src/kwin/plugins/scenes/qpainter/scene_qpainter.cpp:286
#6  0x00007fc9e8f8948d in KWin::Scene::finalDrawWindow(KWin::EffectWindowImpl*, int, QRegion, KWin::WindowPaintData&) (this=this@entry=0x55c403f7b840, w=w@entry=0x55c404899720, mask=mask@entry=10, region=..., data=...) at /home/meven/kde/src/kwin/effects.h:655
#7  0x00007fc9e8eee7e3 in KWin::EffectsHandlerImpl::drawWindow(KWin::EffectWindow*, int, QRegion, KWin::WindowPaintData&) (data=..., region=..., mask=10, w=0x55c404899720, this=0x55c404899720) at /home/meven/kde/src/kwin/effects.cpp:486
#8  0x00007fc9e8eee7e3 in KWin::EffectsHandlerImpl::drawWindow(KWin::EffectWindow*, int, QRegion, KWin::WindowPaintData&) (this=this@entry=0x55c40412c240, w=w@entry=0x55c404899720, mask=mask@entry=10, region=..., data=...) at /home/meven/kde/src/kwin/effects.cpp:480
#9  0x00007fc9e8f8352c in KWin::Scene::finalPaintWindow(KWin::EffectWindowImpl*, int, QRegion, KWin::WindowPaintData&) (this=this@entry=0x55c403f7b840, w=w@entry=0x55c404899720, mask=mask@entry=10, region=..., data=...) at /home/meven/kde/src/kwin/scene.cpp:612
#10 0x00007fc9e8eee61b in KWin::EffectsHandlerImpl::paintWindow(KWin::EffectWindow*, int, QRegion, KWin::WindowPaintData&) (data=..., region=..., mask=10, w=0x55c404899720, this=0x55c40412c240) at /home/meven/kde/src/kwin/effects.cpp:449
#11 0x00007fc9e8eee61b in KWin::EffectsHandlerImpl::paintWindow(KWin::EffectWindow*, int, QRegion, KWin::WindowPaintData&) (this=this@entry=0x55c40412c240, w=0x55c404899720, mask=mask@entry=10, region=..., data=...) at /home/meven/kde/src/kwin/effects.cpp:443
#12 0x00007fc9e8f89341 in KWin::Scene::paintWindow(KWin::Scene::Window*, int, QRegion, KWin::WindowQuadList) (this=this@entry=0x55c403f7b840, w=0x55c4046742a0, mask=10, region=..., quads=...) at /home/meven/kde/src/kwin/scene.cpp:484
#13 0x00007fc9e8f86847 in KWin::Scene::paintSimpleScreen(int, QRegion) (this=this@entry=0x55c403f7b840, orig_mask=orig_mask@entry=8, region=...) at /home/meven/kde/src/kwin/libkwineffects/kwineffects.h:2542
#14 0x00007fc9e8f88890 in KWin::Scene::finalPaintScreen(int, QRegion, KWin::ScreenPaintData&) (this=this@entry=0x55c403f7b840, mask=mask@entry=8, region=..., data=...) at /home/meven/kde/src/kwin/scene.cpp:199
#15 0x00007fc9e8eee375 in KWin::EffectsHandlerImpl::paintScreen(int, QRegion, KWin::ScreenPaintData&) (data=..., region=..., mask=8, this=0x55c403f7b840) at /home/meven/kde/src/kwin/effects.cpp:406
#16 0x00007fc9e8eee375 in KWin::EffectsHandlerImpl::paintScreen(int, QRegion, KWin::ScreenPaintData&) (this=this@entry=0x55c40412c240, mask=8, region=..., data=...) at /home/meven/kde/src/kwin/effects.cpp:400
#17 0x00007fc9e8f8853e in KWin::Scene::paintScreen(int*, QRegion const&, QRegion const&, QRegion*, QRegion*, QMatrix4x4 const&, QRect const&)
    (this=this@entry=0x55c403f7b840, mask=mask@entry=0x7ffe8fc86e04, damage=..., repaint=..., updateRegion=updateRegion@entry=0x7ffe8fc86e10, validRegion=validRegion@entry=0x7ffe8fc86e18, projection=..., outputGeometry=...) at /home/meven/kde/src/kwin/scene.cpp:149
#18 0x00007fc9dd8e5aef in KWin::SceneQPainter::paint(QRegion, QList<KWin::Toplevel*>) (this=this@entry=0x55c403f7b840, damage=..., toplevels=...) at /usr/include/x86_64-linux-gnu/qt5/QtGui/qmatrix4x4.h:358
#19 0x00007fc9e8eb3c91 in KWin::Compositor::performCompositing() (this=0x55c403f6e900) at /home/meven/kde/src/kwin/composite.cpp:693

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Kubuntu 19.04, linux kernel 5.0
KDE Plasma Version:  5.17.80
KDE Frameworks Version: 5.63.0
Qt Version: 5.12.2

Hardware:
Ryzen 5 1600
AMd Radeon RX 560 - amdgpu driver, Mesa 19.0.8