Speeding up simple math operations on numeric 2D array (matrix) in JavaScript

I have a numeric 2D array (an array of arrays, or a matrix) and I need to do simple matrix operations like adding a value to each row, or multiplying every value by a single number. I have little experience with math operations in JavaScript, so this may be a bone-headed code snippet. It is also very slow, and I need to use it when the number of columns is 10,000 - 30,000. By very slow I mean roughly 500 ms to process a row of 2,000 values. Bummer.

var ran2Darray = function(row, col){
    var res = [];
    for (var i = 0 ; i < row; i++) {
        res[i] = [];
        for (var j = 0; j < col; j++) {
            res[i][j] = Math.random();
        }
    }
    return res;
}

var myArray = ran2Darray(5, 100);
var offset = 2;

for (i = 0; i < myArray.length; i++) {
    aRow = myArray[i];
    st = Date.now();
    aRow.map(function addNumber(offset) {myArray[i] + offset*i; })
    end = Date.now();
    document.write(end - st);
    document.write("</br>");
    myArray[i] = aRow;
}

I want to avoid any added libraries or frameworks, unless of course, that is my only option. Can this code be made faster, or is there another direction I can go, like passing the calculation to another language? I'm just not familiar with how people deal with this sort of problem. forEach performs roughly the same, by the way.