Trying to figure out why the View Controller using a MTKView and a Metal-backed CIContext runs at 1/3 of the speed of an GLKView + OpenGLES2-backed CIContext.

MTKViewController.swift

//
//  ViewController.swift
//  MetalCoreImage
//
//  Created by Joshua Sullivan on 4/21/16.
//  Copyright © 2016 The Nerdery. All rights reserved.
//

import UIKit
import MetalKit
import CoreImage

class ViewController: UIViewController {

    private struct FrameRate: CustomStringConvertible {
        let single: Double
        let average: Double
        
        var description: String {
            let formattedString = NSString(format: "single: %0.1f, average: %0.1f", single, average)
            return "FrameRate(\(formattedString))"
        }
    }
    
    private var renderRect: CGRect = CGRect.zero
    
    private var lastTime: CFTimeInterval = 0.0
    
    private var timeAccumulator: CFTimeInterval = 0.0
    
    private var framesRendered: Double = 0.0
    
    private var rotation: CGFloat = 0.0
    
    private var metalDevice: MTLDevice!
    
    private var context: CIContext!
    
    private var blurFilter: CIFilter!
    
    private var transformFilter: CIFilter!
    
    private var commandQueue: MTLCommandQueue!
    
    private var colorSpace = CGColorSpaceCreateDeviceRGB()!
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        guard let device = MTLCreateSystemDefaultDevice() else {
            debugPrint("Unable to create metal device. Aborting.")
            self.view = UIView(frame: view.frame)
            return
        }
        metalDevice = device
        commandQueue = device.newCommandQueue()
        let mView = self.view as! MTKView
        mView.device = device
        mView.delegate = self
        mView.framebufferOnly = false
        
        context = CIContext(MTLDevice: device, options: [kCIContextWorkingColorSpace : NSNull()])
        let checkers = CIFilter(name: "CICheckerboardGenerator")!
        blurFilter = CIFilter(name: "CIGaussianBlur", withInputParameters: [kCIInputImageKey : checkers.outputImage!, kCIInputRadiusKey : 40.0])
        transformFilter = CIFilter(name: "CIAffineTransform", withInputParameters: ["inputImage" : blurFilter.outputImage!])
//        filter = checkers
    }
    
    private func calculateFrameRate() -> FrameRate {
        let now = CACurrentMediaTime()
        guard lastTime > 0.0 else {
            lastTime = now
            return FrameRate(single: 0.0, average: 0.0)
        }
        let dt = now - lastTime
        timeAccumulator += dt
        framesRendered += 1.0
        let averageTime = timeAccumulator / framesRendered
        let averageFrameRate = 1.0 / averageTime
        lastTime = now
        return FrameRate(single: 1.0 / dt, average: averageFrameRate)
    }
}

extension ViewController: MTKViewDelegate {
    func drawInMTKView(view: MTKView) {
        guard let currentDrawable = view.currentDrawable else {
            debugPrint("No currentDrawable.")
            return
        }
        rotation += CGFloat(M_PI / 480.0)
        let blur = sin(rotation * 4.0) * 20.0 + 20.0
        blurFilter.setValue(blur, forKey: kCIInputRadiusKey)
        
        let transform = CGAffineTransformMakeRotation(rotation)
        let boxedTransform = NSValue(CGAffineTransform: transform)
        transformFilter.setValue(boxedTransform, forKey: kCIInputTransformKey)
        transformFilter.setValue(blurFilter.outputImage!, forKey: kCIInputImageKey)
        
        let commandBuffer = commandQueue.commandBuffer()
        let targetTexture = currentDrawable.texture
        
        context.render(transformFilter.outputImage!, toMTLTexture: targetTexture, commandBuffer: commandBuffer, bounds: renderRect, colorSpace: colorSpace)
        commandBuffer.presentDrawable(currentDrawable)
        commandBuffer.commit()
        
        let frameRate = calculateFrameRate()
        debugPrint(frameRate)
    }
    
    func mtkView(view: MTKView, drawableSizeWillChange size: CGSize) {
        renderRect = CGRect(x: 0.0, y: 0.0, width: size.width, height: size.height)
    }
}

OGLViewController.swift

//
//  ViewController.swift
//  OGLCoreImage
//
//  Created by Joshua Sullivan on 4/21/16.
//  Copyright © 2016 The Nerdery. All rights reserved.
//

import UIKit
import GLKit
import CoreImage

class ViewController: UIViewController {
    
    private struct FrameRate: CustomStringConvertible {
        let single: Double
        let average: Double
        
        var description: String {
            let formattedString = NSString(format: "single: %0.1f, average: %0.1f", single, average)
            return "FrameRate(\(formattedString))"
        }
    }
    
    lazy private var glContext: EAGLContext = {
        return EAGLContext(API: .OpenGLES2)
    }()
    
    lazy private var context: CIContext = {
        [unowned self] in
        return CIContext(EAGLContext: self.glContext, options: [kCIContextWorkingColorSpace : NSNull()])
    }()
    
    private var lastTime: CFTimeInterval = 0.0
    
    private var timeAccumulator: CFTimeInterval = 0.0
    
    private var framesRendered: Double = 0.0
    
    private var rotation: CGFloat = 0.0
    
    private var transformFilter: CIFilter!
    
    private var blurFilter: CIFilter!
    
    private var displayLink: CADisplayLink!
    
    private let scaleTransform: CGAffineTransform = {
        let scale = UIScreen.mainScreen().nativeScale
        return CGAffineTransformMakeScale(scale, scale)
    }()
    
    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.
        guard let glView = self.view as? GLKView else {
            assertionFailure("view isn't a GLKView.")
            return
        }
        glView.context = glContext
        glView.delegate = self
        
        let checkers = CIFilter(name: "CICheckerboardGenerator")!
        blurFilter = CIFilter(name: "CIGaussianBlur", withInputParameters: [kCIInputImageKey : checkers.outputImage!, kCIInputRadiusKey : 40.0])
        transformFilter = CIFilter(name: "CIAffineTransform", withInputParameters: ["inputImage" : blurFilter.outputImage!])
        
        displayLink = CADisplayLink(target: self, selector: #selector(displayLinkTick(_:)))
        displayLink.addToRunLoop(NSRunLoop.currentRunLoop(), forMode: kCFRunLoopCommonModes as String)
    }
    
    @objc private func displayLinkTick(link: CADisplayLink) {
        rotation += CGFloat(M_PI / 480.0)
        
        let blur = sin(rotation * 4.0) * 20.0 + 20.0
        blurFilter.setValue(blur, forKey: kCIInputRadiusKey)
        
        let transform = CGAffineTransformMakeRotation(rotation)
        let boxedTransform = NSValue(CGAffineTransform: transform)
        transformFilter.setValue(boxedTransform, forKey: kCIInputTransformKey)
        transformFilter.setValue(blurFilter.outputImage!, forKey: kCIInputImageKey)
        
        
        (self.view as! GLKView).display()
        
        debugPrint("frame rate:", calculateFrameRate())
    }
    
    private func calculateFrameRate() -> FrameRate {
        let now = CACurrentMediaTime()
        guard lastTime > 0.0 else {
            lastTime = now
            return FrameRate(single: 0.0, average: 0.0)
        }
        let dt = now - lastTime
        timeAccumulator += dt
        framesRendered += 1.0
        let averageTime = timeAccumulator / framesRendered
        let averageFrameRate = 1.0 / averageTime
        lastTime = now
        return FrameRate(single: 1.0 / dt, average: averageFrameRate)
    }

}

extension ViewController: GLKViewDelegate {
    func glkView(view: GLKView, drawInRect rect: CGRect) {
        let renderRect = CGRectApplyAffineTransform(rect, scaleTransform)
        context.drawImage(transformFilter.outputImage!, inRect: renderRect, fromRect: renderRect)
    }
}