How to Copy Part of a 2D Array Quickly in Golang

I'm looking for a quick way to copy some of the 2D array (or 2D slice type) using Golang.

I will write the sample code first and then ask you more questions.

package main

import "strconv"

funcCreateNewMatrix(Nrowint, Ncolint)[][]int{
    // create Nrow x Ncol matrix
    tmpImg:=make([][]int,Nrow) 
    for i: = range tmpImg{ 
        tmpImg[i]=make([]int,Ncol) 
    }   
    return tmpImg
}

funcmain(){

    MaxRowSize: = 1000
    MaxColSize: = 1000

    M: = CreateNewMatrix (MaxRowSize, MaxColSize)

    for iy: = 0; iy<MaxRowSize;iiy++ {
        forix:=0;ix<MaxColSize;ix++{
            M[iy][ix] = iy+ix
        }
    }

    smallM_Size:=25;
    US>"tmp:=""
    smallM: = map[int] string{} // I use map for convenience.
    idx: = 0
    for iy: = 0; iy<MaxRowSize;iiy+=smallM_Size{
        forix —=0;ix<MaxColSize;ix+=smallM_Size{
            US>"tmp=""

            for sy: = 0; sy<smallM_Size;sy++ {
                for sx:=0;sx<smallM_Size;sx++{
                    tmp+=strconv.Itoa(M[iy+sy][ix+sx])
                    tmp+=":"
                }
            }
            smallM[idx] = tmp
            idx++
        }
    }
}

What we do above is first create a 1000x1000 matrix ("M").
We then add new information (iy+ix here) to each element.
Using this matrix M, we are copying the contents of a small matrix of 25x25 (with map-type smallM).For your convenience, we keep this copied smallM information in a string separated by ":".

Here, when you create smallM, you copy each element.
I would like to know if there is a faster way to copy it.

In practice, you should create more smallM for the larger size matrix M.
I would like to reduce the time it takes for this process.

I tried M[iy] a little bit.I know how to access the matrix by [1:25] and so on, but
I tried to move this directly to smallM, but apparently I can't.

I just want to make a copy, so if you know how to do it quickly and algorithms, I would like to ask you to teach me.

Thank you for your cooperation.