The optimal solution would be instead if you create a `CollectionType` which repeats the padding character N times. That way you could use `insertContentsOf(_:at:)` to insert N characters at once into the string and move the contents of the string only once.

If that doesn't work for you might as well simply allocate a new `String`, reserve `pad` chars of space, insert the `pad` chars and then insert the previous string. Even that is probably still faster than your previous approach.

In the end it's still probably the easiest approach in practice to simply use the `NSString` method `stringByPaddingToLength(_:withString:startingAtIndex:)`... :D