Histogram of UIImage - calculating data of Lookup Table - swift

Question

Code
This is my code for creating Lookup Table from UIImage - in order to use it as data for image's histogram. But I'm not quite happy with the performance time of it.
extension UIImage {
func getImageLookupTable() -> [UIColor: [Int]] {
let startTime = CFAbsoluteTimeGetCurrent()
guard let cgImage = self.cgImage else { return [:] }
guard let imageData = cgImage.dataProvider?.data as Data? else { return [:] }
let size = cgImage.width * cgImage.height
var LUT: [UIColor: [Int]] = [:]
//Grayscale, single channel image
if cgImage.bitsPerPixel == 8 {
let buffer = UnsafeMutableBufferPointer<UInt8>.allocate(capacity: size)
_ = imageData.copyBytes(to: buffer)
var gray_channel: [Int] = Array(repeating: Int(0), count: 256)
for pixel in buffer {
var grayVal : UInt32 = 0
if cgImage.byteOrderInfo == .orderDefault || cgImage.byteOrderInfo == .order16Little {
grayVal = UInt32((pixel) & 255)
gray_channel[Int(grayVal)]+=1
}
}
LUT[.black] = gray_channel
print(gray_channel)
}
//RGBA or BGRA, 4 channels image
if cgImage.bitsPerPixel == 32 {
let buffer = UnsafeMutableBufferPointer<UInt32>.allocate(capacity: size)
_ = imageData.copyBytes(to: buffer)
var red_channel:[Int] = Array(repeating: Int(0), count: 256)
var green_channel:[Int] = Array(repeating: Int(0), count: 256)
var blue_channel:[Int] = Array(repeating: Int(0), count: 256)
for pixel in buffer {
var r : UInt32 = 0
var g : UInt32 = 0
var b : UInt32 = 0
if cgImage.byteOrderInfo == .orderDefault || cgImage.byteOrderInfo == .order32Big {
r = pixel & 255
red_channel[Int(r)]+=1
g = (pixel >> 8) & 255
blue_channel[Int(g)]+=1
b = (pixel >> 16) & 255
green_channel[Int(b)]+=1
}
else if cgImage.byteOrderInfo == .order32Little {
r = (pixel >> 16) & 255
g = (pixel >> 8) & 255
b = pixel & 255
blue_channel[Int(b)]+=1
green_channel[Int(g)]+=1
red_channel[Int(r)]+=1
}
}
LUT[.red] = red_channel
LUT[.blue] = blue_channel
LUT[.green] = green_channel
}
let timeElapsed = CFAbsoluteTimeGetCurrent() - startTime
print("Time elapsed for \(self.size): \(timeElapsed) s.")
return LUT
}
}
Problem
It takes around 10.15s for image (RGBA) with size: 3024 x 4032, which is definitely to much.
How can I improve it, what would you recommend ?