The issue wasn't that enable compositor wasn't checked it was that KWin was not using OpenGL any more. So although looking into this menu was the critical step towards the solution it wasn't the solution itself, so if someone else with the same problem would look at the post, this would not solve it.
(Of course all the credit goes to @anon72786180 lead me onto the right track!)
(If you have a point system for solutions, Catra could also edit her answer to make it the solution and get credit this way)