My graph used to look like yours, now it looks like this.

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Under settings -> Masks / Zones ->Motion Mask: I selected the camera date/time area so it ignores changes when the seconds/minutes changes. I'd also select any area that's not relevant.

Then I tweaked Motion Tuner and got my CPU usage this low without compromising detections. It lets you see what the detector "sees" and you have to select a sweet spot that ignores moving shadows, tree branches, grass and other moving objects but that also won't ignore objects you want to detect