How can I optimize my code for better execution times - opencv

Question

I'm implementing an image processing algorithm called BM3D and I've already achieved the outcome which is denoising a grayscale image but the thing is that it is too slow, even with a 436 by 436 gray image.
I have already tried to find way to maybe fasten up the work that I do with array and lists, but didn't get much improvement
val img = imread("files/image.png", 0)
val img3= Mat(img.rows(),img.cols(),img.type())
val listaBlocos = mutableListOf(Pair(0.0, Pair(0,0)))
val tamanhoBloco = 3 //Block Size
val tamanhoJanela = 9 //Window Size
val mediaPorBloco = DoubleArray(16)
var sum = 0.0
listaBlocos.clear()
val stats_file = File("files/tempos436x436.txt")
val test = 10
for (x in 0 until test){
val timeelapsed = measureTimeMillis {
for (col in 20 ..img.width() - 20) {
for (row in 20 ..img.height() - 20) {
val block1 = generateBlock(img, row, col, tamanhoBloco)
for (c in -tamanhoJanela..tamanhoJanela) {
for (l in -tamanhoJanela..tamanhoJanela) {
val block2 = generateBlock(img, row + l, col + c, tamanhoBloco)
val d = distBlock(block1, block2)
val par = Pair(d, Pair(row + l, col + c))
listaBlocos.add(par)
}
}
val listaBlocosOrdenada = listaBlocos.sortedWith(compareBy { it.first })
listaBlocos.clear()
for (k in 0..15) {
sum = 0.0
val c2 = listaBlocosOrdenada[k].second.second
val l2 = listaBlocosOrdenada[k].second.first
for (c in 0..tamanhoBloco - 1) {
for (l in 0..tamanhoBloco - 1) {
sum += img.get(l2 - l, c2 - c)[0]
}
}
mediaPorBloco[k] = sum / 4
}
val v = mediaPorBloco.average()
img3.put(row,col,v)
}
}
}
imwrite("files/resultado.png", img3)
stats_file.appendText("teste$x 100X200 $timeelapsed\n")
}
well the result in the actual image denoising is good but is takes maybe 15 min to denoise a 436 x 436 image. I'm currently using a virtual machine with Ubuntu and 4 cores a 4 gbs of Ram