SpriteKit快速移动帧速率下降

Question

我一直致力于一项涉及SpriteKit快速移动的游戏。我已经对游戏进行了测试和分析，删除了游戏的多个部分，并且无法保持稳定的60 fps。帧速率下降导致节点移动到断断续续。我决定建立一个简单的程序进行测试。屏幕上只有4个节点从左向右移动，iPhone 7 Plus上运行的帧数仍有下降。有没有办法优化SpriteKit中的快速移动以保持稳定的帧速率？

import SpriteKit
import GameplayKit

class GameScene: SKScene {

var ship = SKSpriteNode()
var ship2 = SKSpriteNode()
var ship3 = SKSpriteNode()
var ship4 = SKSpriteNode()

override func didMove(to view: SKView) {

    ship.texture = SKTexture(imageNamed: "Spaceship")
    ship.position = CGPoint(x: -300, y: 0)
    ship.size = CGSize(width: 50, height: 50)
    ship.setScale(1)
    ship.physicsBody = SKPhysicsBody(rectangleOf: ship.size)
    ship.physicsBody?.affectedByGravity = false
    ship.zPosition = 2
    self.addChild(ship)

    ship2.texture = SKTexture(imageNamed: "Spaceship")
    ship2.position = CGPoint(x: -200, y: -50)
    ship2.size = CGSize(width: 50, height: 50)
    ship2.setScale(1)
    ship2.physicsBody = SKPhysicsBody(rectangleOf: ship.size)
    ship2.physicsBody?.affectedByGravity = false
    ship2.zPosition = 2
    self.addChild(ship2)

    ship3.texture = SKTexture(imageNamed: "Spaceship")
    ship3.position = CGPoint(x: -100, y: 50)
    ship3.size = CGSize(width: 50, height: 50)
    ship3.setScale(1)
    ship3.physicsBody = SKPhysicsBody(rectangleOf: ship.size)
    ship3.physicsBody?.affectedByGravity = false
    ship3.zPosition = 2
    self.addChild(ship3)

    ship4.texture = SKTexture(imageNamed: "Spaceship")
    ship4.position = CGPoint(x: -0, y: 100)
    ship4.size = CGSize(width: 50, height: 50)
    ship4.setScale(1)
    ship4.physicsBody = SKPhysicsBody(rectangleOf: ship.size)
    ship4.physicsBody?.affectedByGravity = false
    ship4.zPosition = 2
    self.addChild(ship4)

}

override func touchesBegan(_ touches: Set<UITouch>, with event:     UIEvent?) {

}

override func touchesEnded(_ touches: Set<UITouch>, with event: UIEvent?) {

}


override func update(_ currentTime: TimeInterval) {

    // Called before each frame is rendered
    ship.position.x += 15
    ship2.position.x += 15
    ship3.position.x += 15
    ship4.position.x += 15

    if ship.position.x > 800 {
        ship.position.x = -950
    } else if ship2.position.x > 800 {
        ship2.position.x = -900
    } else if ship3.position.x > 800 {
        ship3.position.x = -850
    } else if ship4.position.x > 800 {
        ship4.position.x = -800
    }

}

}

编辑：在分析测试程序时，帧速率降至59 fps。