premature optimization

test1.js

var pool = {
    query: function(sql, callback) {
        process.nextTick(function() {
            var result = [
                {0:12345, 1:'EUR/USD', 2:1.2234, 3:1.2221, 4:1.2240, length: 5},
                {0:12345, 1:'EUR/USD', 2:1.2234, 3:1.2221, 4:1.2240, length: 5 },
                {0:12345, 1:'EUR/USD', 2:1.2234, 3:1.2221, 4:1.2240, length: 5}]
            callback(null, result);
        });
    }
};

function castArray(result) {
    var len = result.length, arr = [], x;
    for (x = 0; x < len; x += 1) {
        arr[x] = Array.prototype.slice.call(result[x]);
    };
    return arr;
}

var start1 = process.hrtime();
pool.query('SELECT id AS `0`, symbol AS `1`, price AS `2`, bid AS `3`, ask AS `4`, 5 AS `length` FROM session_quotes', function (error, result) {
if (error) {
    console.log(error);
    return;
};
    result = castArray(result);
    console.log("time:" + process.hrtime(start1));
    console.log(result);
});

test2.js

var pool = {
    query: function(sql, callback) {
        process.nextTick(function() {
            var result = [
                {id:12345, symbol:'EUR/USD', price:1.2234, bid:1.2221, ask:1.2240},
                {id:12345, symbol:'EUR/USD', price:1.2234, bid:1.2221, ask:1.2240 },
                {id:12345, symbol:'EUR/USD', price:1.2234, bid:1.2221, ask:1.2240}]
            callback(null, result);
        });
    }
};

var start1 = process.hrtime();
pool.query('SELECT id, symbol, price, bid, ask FROM session_quotes', function (error, result) {
if (error) {
    console.log(error);
    return;
};
    console.log("time:" + process.hrtime(start1));
    console.log(result);
});

а зачем нужен тут nextTick?

Он не нужен, но и не мешает. Просто для симуляции ответа от базы данных.