Do functions decrease performance? Answer: No. Notice the extra let obj=[].

functions-vs-no-functions.js

var debug = require('debug')('test');

function sum(a, b) {
    return a + b;
}

function multiplyTwo(a) {
    return a * 2;
}

function divideByTwo(a) {
    return a / 2;
}

function subtractOne(a) {
    return a - 1;
}

function runOne(a, times) {
    let obj = [];
    //with functions
    for (let i = 0; i < times; i++) {
        a = sum(a, 1);
        a = multiplyTwo(a);
        a = divideByTwo(a);
        a = subtractOne(a);
    }
    debug('end with functions ' + a);
}

function runTwo(a, times) {
    let obj = [];
    //without functions
    for (let i = 0; i < times; i++) {
        a = a + 1;
        a = a * 2;
        a = a / 2;
        a = a - 1;
    }
    debug('end without functions ' + a);
}

const times = 900000000;

debug('start with functions');
runOne(2, times);
debug('start without functions');
runTwo(2, times);

/*
results:
/small$ DEBUG=* node a.js
  test start with functions +0ms
  test end with functions 2 +10s
  test start without functions +3ms
  test end without functions 2 +10s
*/