I’ve seen (and posted myself) rumblings about this a couple times with no clear answer. I get a noticeably higher (like three times) CPU load according to the AUM load meter when an AUv3’s UI is closed than when it is open. This is both release and debug builds, and has been happening pretty much since we started releasing iOS AUv3. It is not limited to JUCE, either. I’ve noticed that it happens on, for instance, the AudioKit synths, as well.
I’ve always assumed that since the Editor always exists in an AUv3 on iOS, rather than getting deleted when it is closed like on desktop, this was the result of background drawing. Today, I wrapped every single paint and timer in my editor in an isShowing() if, and it had no effect whatsoever. (Despite the fact that the UI is backgrounded, isShowing() correctly reports whether it is “open” to the user or not.)
Does anyone have any idea as to the cause of this, and what might mitigate it? Or is it just a quirk of AUM that their DSP meter (and yes, I’m aware that it is nearly impossible to actually meter a plugin’s load) shows a higher load when the UI isn’t showing?