Tips for fast string concatenation?

[Gary Willoughby]

Have you any tips for using D when you need fast string 
concatenation? I regularly use code like this:

    foreach (i, range)
    {
        foo ~= bar;
    }
or:

    foo = foo ~ bar ~ baz ~ qux;

I've used std.string.format(...) in some instances which sped 
things up which surprised me.

Are there faster ways of appending strings?