I made some more tests today with two other window managers: Awesome and Openbox
With openbox I get the Renoise issue without Compton and even with Compton. And even more funny, only when the Renoise window is 100% maximized. When there´ s a small frame of the desktop left it works, i.e. no higher DSP load. Under XFCE the DSP load was more or less proportional to the visible Renoise GUI.
In Awseome it works even without Compton.