The "suspend compositor for full screen windows" option used to suspend it only while the full screen window was active. Now it is suspended while there is a full screen window even if it is in the background (minimized). Reproducible: Always
The setting is irrelevant (and works differently and not at all on intel chips), the window asks to avoid the compositor. See dupe and notably https://bugs.kde.org/show_bug.cgi?id=361091#c3 on how to override the (or all) clients opinion. *** This bug has been marked as a duplicate of bug 361091 ***
OK, I see that kwin now uses the _NET_WM_BYPASS_COMPOSITOR hint. In my opinion that is a stupid thing to do. For example, Team Fortress 2 disables the compositor while it's running, even if it's in the background, but Garry's Mod doesn't disable it at all. I mostly use composite effects for window management, so disabling the compositor also deceases usability for me.