It seems for the moment string concatenation is faster: http://jsperf.com/es6-string-literals-vs-string-concatenation

ES6 with variable                     19,992,512    ±5.21%    78% slower

String concatenation with variable    89,791,408    ±2.15%    fastest

ES6 with function                     461,358       ±3.12%    99% slower

String concatenation with function    503,255       ±1.77%    99% slower

I tested was run on Chrome 43.0.2334.0 canary (64-bit), which is using V8 4.3.31, with the #enable-javascript-harmony flag enabled.

For reference, the latest version on Node.js (0.12.0 at the time of writing) is using V8 3.28.73: https://raw.githubusercontent.com/joyent/node/master/ChangeLog

I'm sure all the possible performance optimizations that could be applied have not been applied yet, so it would be reasonable to expect performance to get better as ES6 gets closer to finalization and these features get migrated to the stable branch.

Edit: Thanks for the comments @user1329482, @icl7126, Nicolai Borisik, and FesterCluck. Now that about 2 years have passed since this question was asked, ES6 browser support has greatly increased, and a good amount of performance optimization has taken place. Here are some updates.

Edit: (February 2020) Updated Chrome result based on @JorgeFuentesGonzález comments and subsequent confirmation.

In Chrome (as of 59.0.3035), ES6 string literals are faster:

ES6 with variable                     48,161,401       ±1.07%    fastest

String concatenation with variable    27,046,298       ±0.48%    44% slower

ES6 with function                     820,441          ±1.10%    98% slower

String concatenation with function    807,088          ±1.08%    98% slower

Update: In Chrome (as of 79.0.3945), String concatenation is faster... See comments.

In Firefox (as of 57.0.0), ES6 string literals are faster:

ES6 with variable                     1,924,610,984    ±0.50%    fastest

String concatenation with variable    1,876,993,458    ±0.79%    3% slower

ES6 with function                     539,762          ±5.04%    100% slower

String concatenation with function    546,030          ±5.88%    100% slower

In Safari (as of 11.0.2), it depends:

ES6 with variable                     1,382,752,744    ±0.71%    fastest

String concatenation with variable    1,355,512,037    ±0.70%    2% slower

ES6 with function                     876,516          ±1.01%    100% slower

String concatenation with function    883,370          ±0.79%    100% slower

When using a typecast string, ES6 string literals are faster. However, when calling a function from the literal, string concatenation is faster in this example.

If you really want to go deep and need to squeeze every drop of performance out of Safari, I would suggest setting up tests that see if/how incorrectly typed variables and multiple references within a literal effect performance.