fps.swift

class InterfaceController: WKInterfaceController {
  @IBOutlet weak var iScene: WKInterfaceSKScene!
  var scene:SKScene!
  var t = 0
  
      override func awake(withContext context: Any?) {
        super.awake(withContext: context)
        scene = SKScene(fileNamed: "MyScene.sks")
        iScene.presentScene(scene)
        scene.delegate = self
        
        Timer.scheduledTimer(withTimeInterval: 1, repeats: true) { (timerObj) in
            print("fps = \(self.t)")
            self.t = 0
        }
      }
}

extension InterfaceController: SKSceneDelegate {
    
    func update(_ currentTime: TimeInterval, for scene: SKScene) {
        t += 1
    }
}

Get Frames Per Second log on WatchKit/WatchOS.