bench-getValue.user.js

// ==UserScript==
// @name        test
// @namespace   https://github.com/the8472
// @include     *
// @version     1
// @grant       GM_setValue
// @grant       GM_getValue
// @run-at          document-end
// ==/UserScript==

'use strict';

GM_setValue("foo", "bar");

const iters = 10000;

function bench() {
  let start = performance.now();

  for(let i=0;i<iters;i++) {
    GM_getValue("foo")
  }

  let duration = performance.now() - start;

  console.log(duration)  
}

for(let j=0;j<10;j++) {
  bench();  
}