GetPixel makes the game slow - unity3d

Question

I'm creating a cleaning game but in the getpixel line of code makes the game slow or having a delay. I need this line of code to detect the remaining dirt. How can I improve the code? or is there other way to detect remaining dirt?
private void Update()
{
if (Input.touchCount > 0)
{
Touch touch = Input.GetTouch(0);
if (touch.phase == TouchPhase.Began)
{
if (touch.position != currentPosition || touch.position != prevPosition)
{
SetSinglePixel(touch.position);
prevPosition = touch.position;
}
}
else if (touch.phase == TouchPhase.Moved)
{
if (touch.position != currentPosition || touch.position != prevPosition)
{
currentPosition = touch.position;
Vector2 dif = currentPosition - prevPosition;
float distance = dif.magnitude;
if (distance > 36)
{
float count = distance / 36;
float n = distance / (count + 1);
float ddmo = 36;
for (int i = 0; i <= count; i++)
{
Vector2 gapPoint = Vector2.MoveTowards(prevPosition, currentPosition, ddmo * i);
if (Physics.Raycast(Camera.main.ScreenPointToRay(gapPoint), out RaycastHit hit))
{
gapPositions.Add(hit.textureCoord);
}
}
SetMultiplePixel(gapPositions);
gapPositions.Clear();
}
else
{
SetSinglePixel(currentPosition);
}
prevPosition = touch.position;
}
}
}
}
private void SetMultiplePixel(List<Vector2> givenPoints)
{
foreach (Vector2 pos in givenPoints)
{
SetPixel(pos.x, pos.y);
}
}
private void SetSinglePixel(Vector2 touchPos)
{
if (Physics.Raycast(Camera.main.ScreenPointToRay(touchPos), out RaycastHit hit))
{
SetPixel(hit.textureCoord.x, hit.textureCoord.y);
}
}
public void ResetTexture()
{
SceneManager.LoadScene(SceneManager.GetActiveScene().name);
}
private void SetPixel(float textureCoordX, float textureCoordY, bool isCheckRemainingTexture = false)
{
int pixelX = (int)(textureCoordX * _templateDirtMask.width);
int pixelY = (int)(textureCoordY * _templateDirtMask.height);
int pixelXOffset = pixelX - (_brush.width / 2);
int pixelYOffset = pixelY - (_brush.height / 2);
for (int x = 0; x < _brush.width; x++)
{
for (int y = 0; y < _brush.height; y++)
{
data = (pixelXOffset + x) + (pixelYOffset + y) * _templateDirtMask.width;
if (data > 0 && data < pixels.Length)
{
pixelDirt = _brush.GetPixel(x, y);// this code makes the game slow
Color pixelDirtMask = _templateDirtMask.GetPixel(pixelXOffset + x, pixelYOffset + y); // this code makes the game slow
float removedAmount = pixelDirtMask.g - (pixelDirtMask.g * pixelDirt.g);
dirtAmount -= removedAmount;
pixels[data] = new Color(0, pixelDirtMask.g * pixelDirt.g, 0);
}
}
}
ApplyTexture();
}