benchmark.js

#!/usr/bin/node

const N = 1000000;

// 130 ms
(function() {
  console.time('+=');
  let a = '';
  for (let i = 0; i < N; i++) {
    a += 'x';
  }
  console.timeEnd('+=');
})();

// 70 ms
(function() {
  console.time('= +');
  let a = '';
  for (let i = 0; i < N; i++) {
    a = a + 'x';
  }
  console.timeEnd('= +');
})();

V8 5.9 with turbofan. The generated optimized code looks the same to me. Seems like the first function is always the slower one. So += is perfectly fine to use in the future. You can download Node with newest V8 for Ubuntu from here.

~/code/JoLissExample$ ../../Downloads/bin/node -e "console.log(process.versions.v8)"
5.9.0 (candidate)
~/code/JoLissExample$ ../../Downloads/bin/node  loop.js
= +: 108.032ms
+=: 66.177ms
= +: 78.599ms

Old V8 (Node 7) with crankshaft:

~/code/JoLissExample$ node --version
v7.7.2
~/code/JoLissExample$ node -e "console.log(process.versions.v8)"
5.5.372.41
~/code/JoLissExample$ node loop.js
= +: 73.538ms
+=: 103.824ms
= +: 28.473ms