I'm looking for some pointers on making this more efficient. It's failing due to it taking too long to execute the 5500+ tests that this Kata runs.

function transposeTwoStrings(arr) {
var twoStrings = '';
var string1 = arr[0];
var string2 = arr[1];
var longestString = arr.sort(function(a, b) {
return b.length - a.length;
})[0].length;

for (i = 0; i < longestString; i++) {
twoStrings += (string1[i] || ' ') + ' ' + (string2[i] || ' ') + '\n';
}
return twoStrings.slice(0, -1);
}