StringBuilder was "faster"
Are people still suggesting that machine cycles represent the optimal metric of efficiency? Since I started programming in the 1980's people have been making that up hill argument. Isn't an extra $25 toward a better processor a great trade off against me having to maintain the first code sample above.
In short, to me faster must be measured in my time, not the computer's time. The vacuum tube days of catering to the time demands of the machines should, by now, be long gone.