parallelGet.js

console.time("test");
Promise.all([
	new Promise(r=>chrome.storage.sync.get(null, s=>r(s))),
	new Promise(r=>chrome.storage.local.get(null, l=>r(l)))
]).then(x=>console.timeEnd("test"));

console.time("test2");
chrome.storage.sync.get(s=>{
	chrome.storage.local.get(l=>console.timeEnd("test2"));
});

Promise.race([
  Promise.all([
	  new Promise(r=>chrome.storage.sync.get(null, s=>r("Parallel"))),
	  new Promise(r=>chrome.storage.local.get(null, l=>r("Parallel")))
  ]),
  new Promise(r=>{
    chrome.storage.sync.get(null, s=>{
	    chrome.storage.local.get(null, l=>r("Synchronous"));
    });
  })
]).then(x=>console.log(x));