SceneKit: improve performance of SceneView's hitTest function?

Question

Our scene only has about 20 nodes. The code below lets the user pan, conducting a hit test on each pan -- the goal is to highlight blocks as the user pans around the screen.

However, it is noticeably sluggish on a iPhone 5S. It's not deterministic but happens often enough to be irritating (every 5-10 pans).

We considered using hitTestWithSegment since you could tightly bound the range for testing but believe that should be slower because you must first compute the two points required for the function.

Moreover, the SCNHitTestClipToZRangeKey option for hitTest should provide a comparable performance boost by tightening the hit range without requiring the computation of two additional points.

Any suggestions for speeding up the performance of hitTest?

func sceneViewPannedOneFinger(sender: UIPanGestureRecognizer) {
    // Get pan distance & convert to radians
    let translation = sender.translationInView(sender.view!)
    var xRadians = GLKMathDegreesToRadians(Float(translation.x))
    var yRadians = GLKMathDegreesToRadians(Float(translation.y))

    // Get x & y radians
    xRadians = (xRadians / 4) + curXRadians
    yRadians = (yRadians / 4) + curYRadians

    // Limit yRadians to prevent rotating 360 degrees vertically
    yRadians = max(Float(-M_PI_2), min(Float(M_PI_2), yRadians))

    // Set rotation values to avoid Gimbal Lock
    cameraNode.rotation = SCNVector4(x: 1, y: 0, z: 0, w: yRadians)
    userNode.rotation = SCNVector4(x: 0, y: 1, z: 0, w: xRadians)

    // Save value for next rotation
    if sender.state == UIGestureRecognizerState.Ended {
        curXRadians = xRadians
        curYRadians = yRadians
    }

    // Set preview block
    setPreviewBlock(sender)
}


private func setPreviewBlock(recognizer: UIGestureRecognizer) {        
    let point = recognizer.locationInView(sceneView)
    let options = [SCNHitTestRootNodeKey: sceneView.scene!.rootNode, SCNHitTestClipToZRangeKey: 15, SCNHitTestSortResultsKey: true]
    let hits = sceneView.hitTest(point, options: options)
    print(hits.first?.worldCoordinates)
}