Selecting All Options In A Dropdown

I have a drop down with alot of options say about 6000 cities and
districts. I need to select all options if user selects "All checkbox".
I use "on change" and a for loop to do this, but it takes a lot of time
to select all these options. Is there any faster way to select all
these options?

function selectAll()
{
var element = document.getElementById("location");
var length = document.getElementById("location").length;

for(var i=0; i<length; i++)
{
element.options[i].selected=true;
}
}