I frequently see compositor questions for KDE users. People want something beyond the keyboard toggle of Alt+Shift+F12 (There is also a nice widget for this, if you prefer).
It is possible to set a window rule for a specific app to turn off compositing for just an app when it launches. Often gamers want this to increase the frame rate for the game.
Recently I wanted to do this for when I run the legacy Photoshop 7 via WINE. It has a bunch of sub menus that are transparent unless I turn compositing off, which I find annoying. I used to just use the keyboard command. But now I do not need to remember to do this.
Now on to the window rule:
Make sure the box is checked to Allow applications to block compositing. It is in the compositor section of Display and Monitor.
Go to Window Management > Window Rules. Click on the plus sign to create a window rule for the app you want blocked compositing for.
Fill in the description field. In the blank area of Window class (application) type in the application name you want. In my case because it is a WINE app I typed photoshop.exe wine.
Click on the plus sign for Add Property. Start typing the word block until you see Block compositing in Appearance and Fixes. Click on the plus sign to add that property. Click Apply to apply the changes and you are done.