I don't think that's a KDE problem.

kworker => kernel... (not KDE ;-) )

system is that slow because of it, tracing the exact cause is almost impossible , trying a fresh install and update see if that helps

ok so after quite a bit of messing around and it taking up to 5 mins to get the mouse to move from one side of the screen to the other 
i got one window open runing a minecraft server  (no lag or issues ) another streeming online radio again no lag issues , but the gui is imposiblay slow to use 
and after tring to get logs out of system d hope this helps 

Nov 18 04:02:10 linux-icug plasmashell[2429]: trying to show an empty dialog 
Nov 18 04:02:05 linux-icug plasmashell[2429]: org.kde.plasmaquick: Increasing score for "Application Menu" to 100 
Nov 18 04:02:05 linux-icug plasmashell[2429]: org.kde.plasmaquick: Applet "Application Menu" loaded after 0 msec 
Nov 18 04:02:04 linux-icug plasmashell[2429]: qt.qpa.xcb: QXcbConnection: XCB error: 2 (BadValue), sequence: 26981, resource id: 60817432, major code: 142 (Unknown), minor code: 3 
Nov 18 04:00:52 linux-icug plasmashell[2429]: KActivitiesStats( 0x560de5c5acb0 ) ResultModelPrivate::onResultScoreUpdated  result added: "applications:org.kde.konsole.desktop" score: 10.3225 last: 1542513649 first: 1493986679 
Nov 18 04:00:49 linux-icug kwin_x11[2420]: qt.qpa.xcb: QXcbConnection: XCB error: 3 (BadWindow), sequence: 53191, resource id: 37749009, major code: 18 (ChangeProperty), minor code: 0 
Nov 18 04:00:36 linux-icug plasmashell[2429]: trying to show an empty dialog 
Nov 18 04:00:33 linux-icug plasmashell[2429]: org.kde.plasmaquick: Increasing score for "Application Menu" to 100 
Nov 18 04:00:33 linux-icug plasmashell[2429]: org.kde.plasmaquick: Applet "Application Menu" loaded after 0 msec 
Nov 18 03:51:35 linux-icug plasmashell[2429]: KActivitiesStats( 0x560de5c5acb0 ) ResultModelPrivate::onResultScoreUpdated  result added: "applications:org.kde.konsole.desktop" score: 9.32253 last: 1542513092 first: 1493986679 
Nov 18 03:51:32 linux-icug kwin_x11[2420]: qt.qpa.xcb: QXcbConnection: XCB error: 3 (BadWindow), sequence: 57550, resource id: 37749004, major code: 18 (ChangeProperty), minor code: 0 
Nov 18 03:51:23 linux-icug plasmashell[2429]: trying to show an empty dialog 
Nov 18 03:51:16 linux-icug plasmashell[2429]: trying to show an empty dialog 
Nov 18 03:47:38 linux-icug kwin_x11[2420]: qt.qpa.xcb: QXcbConnection: XCB error: 3 (BadWindow), sequence: 39954, resource id: 83886085, major code: 18 (ChangeProperty), minor code: 0 
Nov 18 03:45:10 linux-icug plasmashell[2429]: qt.svg: link y hasn't been detected! 
Nov 18 03:45:10 linux-icug plasmashell[2429]: qt.svg: link y hasn't been detected! 
Nov 18 03:45:10 linux-icug plasmashell[2429]: qt.svg: link y hasn't been detected! 
Nov 18 03:45:10 linux-icug plasmashell[2429]: qt.svg: link x hasn't been detected! 
Nov 18 03:45:10 linux-icug plasmashell[2429]: qt.svg: link y hasn't been detected! 
Nov 18 03:45:10 linux-icug plasmashell[2429]: qt.svg: link y hasn't been detected! 
Nov 18 03:45:10 linux-icug plasmashell[2429]: qt.svg: link y hasn't been detected! 
Nov 18 03:45:10 linux-icug plasmashell[2429]: qt.svg: link y hasn't been detected! 
Nov 18 03:45:10 linux-icug plasmashell[2429]: qt.svg: link y hasn't been detected! 
Nov 18 03:45:10 linux-icug plasmashell[2429]: qt.svg: link y hasn't been detected! 
Nov 18 03:45:10 linux-icug plasmashell[2429]: qt.svg: link x hasn't been detected! 
Nov 18 03:45:10 linux-icug plasmashell[2429]: qt.svg: link x hasn't been detected! 
Nov 18 03:45:03 linux-icug kwin_x11[2420]: qt.qpa.xcb: QXcbConnection: XCB error: 3 (BadWindow), sequence: 27921, resource id: 81788988, major code: 18 (ChangeProperty), minor code: 0 
Nov 18 03:45:03 linux-icug kwin_x11[2420]: qt.qpa.xcb: QXcbConnection: XCB error: 3 (BadWindow), sequence: 27917, resource id: 77594629, major code: 18 (ChangeProperty), minor code: 0

(In reply to david powell from comment #0)
> gtx1050Ti graphics card

Are you using the nvidia driver by chance?

At least some versions fail to build with the latest kernel, see https://lists.opensuse.org/opensuse-factory/2018-11/msg00082.html.
And if the nvidia kernel module cannot be loaded, the system will fall back to a different driver (nouveau, modesetting, or even fbdev if booting with nomodeset) which can/will be slower. Especially if Mesa-dri-nouveau is not installed, as Plasma does use OpenGL.

So please attach your Xorg log, I'd say.

Created attachment 790073 [details]
xorg log file

update , from install using default installed drivers the problem still persists , with snapshot build 20181126 , 

system settings> display and monitor > compositor from opengl2.0 to xrender made it a bit more usable , but was worse with opengl3.1  although it took a few restarts to get the mouse pointer back

this is using the drivers installed by default with the snapshot 

i installed the official 3rd party nvidia drivers and the problem seems to have gone , although my system now works fine so far this does not resolve the issue with the default driver    

so i can at least conclude its a driver issue if nothing else

added notes :- some observations of the problem , 

on some restarts the problem did not show its self straight away and the display appeared fine for a few mins , although opening chrome and try playing a youtube video and the screen started locking  flickering and such , even after closing the app even if not the problem would show at some point when using apps that dont depend on native opengl functions 

on other restarts the problem would start at the kde login page and continue from that point

more like some data sync problem between threads that may go out of phase over time causing some data to be dropped or over run 

this was true if eather a phisical power down or just a soft reboot 

as to if its related to the driver version problem https://lists.opensuse.org/opensuse-factory/2018-11/msg00082.html.

all i can say is its the current snapshot i am using not the factory version so i would assume that issue would of been addressed before release to the tumbleweed snapshot 

hope this helps

If you are brave enough, you can report this problem to the nouveau driver bugzilla at:
https://bugs.freedesktop.org/enter_bug.cgi

I am afraid, we cannot do much here.