ofVideoGrabber performance issues on iOS

I'm using a simple ofVideoGrabber like this:

void ofApp::setup(){
    _videoGrabber.setup(1280, 720, OF_PIXELS_BGRA);
    _screenWidth = ofGetScreenWidth();
    _screenHeight = ofGetScreenHeight();
}

void ofApp::update(){
    _videoGrabber.update();
}

void ofApp::draw(){
    _videoGrabber.draw(0, 0, _screenWidth, _screenHeight);
}

On an iPhone 6 running iOS 9.1.3, the grabber starts smoothly, but the frame rate gets really bad after 5 seconds or so. Locking the phone and unlocking it restores the smooth frame rate, but it slows down again after further 5 seconds.

Start by reproducing the reported ofVideoGrabber behavior on an iPhone 6 running iOS 9.1.3, using the setup, update, and draw calls shown in the issue. Measure the frame rate before and after roughly five seconds and after locking and unlocking the phone; completion would require identifying and resolving the slowdown, with the frame rate remaining stable.