function sum(a, b, callback) {
    process.nextTick(function(){
        callback(null, a + b);
    });
}
Simple benchmark which compares eventloop speed with Fibers sync

Note: Using process.nextTick in sum() function is not represents the real
      situation. In real live our we use I/O with external sources like
      network or fs. There's no need to use Sync/Fibers with code without I/O.
      So, even if we change process.nextTick to setTimeout() on 3 ms, the result
      will be almost same as native one, because asynchronous function time will
      seriously reduce Sync/Fibers wrapper cost.

      The more I/O in your app, the cheaper cost of Fibers.

On Macbook Pro | 2.66 GHz i7 | DDR3 1067 MHz | OSX 10.7.3 | node v0.6.18, node-fibers v0.6.8

    Event-loop took 163 ms
    Sync took 486 ms (x2)
    Futures took 7216 ms (x44)
    async() took 542 ms (x3)
    async().sync() took 468 ms (x2)
    Fibers.future took 1452 ms (x8)
    Fiber took 422 ms (x2)

REAL result:

    Event-loop took 354 ms
    Sync took 361 ms (x1)
    Futures took 370 ms (x1)
    async() took 353 ms (x0)
    async().sync() took 351 ms (x0)
    Fibers.future took 350 ms (x0)
    Fiber took 350 ms (x0)