Warning: it might be either kwin or overall plasmashell problem, or even something else. I don't have the knowledge to say for sure.
Also the problems started since one of previous updates, today cannot say which exactly as I was waiting for it to be fixed as one of apparent problems when using the system. Possible it's a problem of upgrading from 5.9 to 5.10

Also I set severity to major as that bug is really annoying in everyday usage of the system - a big flaw on user experience and performance.



Problem appears when using apps like Spotify (the worst case), Telegram, Dolphin, Krita and possibly many more I haven't ever noticed or used.

Spotify case: after a new song is played either automaticly or by the user, app does not response for a period of 25 seconds, allowing more or less actions. Scrolling through music playlists is usually possible, but no way to change the song playing, stop it... You need to wait these 25 seconds. In that time the album cover doesn't refresh either.
Happens all the time till the end of undescribed period, probably long hours of the system running, it disappears discarding other problems as well

Telegram case: entering settings runs partially animation of the button, then freezes (I don't know exact time here, but it is most likely related to Spotify case, so to this bug. It is around 20 seconds for sure, expect 25)
Happens just one time after running the system.

Doplhin & Krita case: Any kind of confirmation windows should probably be included here: mostly file deletion is my case. The window doesn't even fully render before the freeze passes - visible are just decorations and possibly some kind of a shadow..?

Discover case: cannot make it a firm example, because it was having problems since I've started using Plasma. Still: possible a freeze after updating system visible when trying to close the app might be the same thing as above.



I might also mention I have created another user account once in the past. Weirdly the other user account doesn't show any signs of the problems listed above. This makes me suspect it is kind of corruption which appears after some time of using one account for some time. And again: it's just my guess - a possible hint.

Another possible hint: after one of updates (cannot say what was updated that time, it was a list of packages, not an update of Plasma though) the problem disappeared for one run of the system. And it came back.
I own a flawed WD hard drive which started delaying my downloads and making Spotify go silent, so I disconnected it from my computer. In effect after running system the delay decreased to 5 seconds, but it didn't last for long, probably matter of 10 minutes and everything is back to "normal" - 25 seconds.