JSFiddle - React, Tailwind, and code Playground

JavaScript

let from = window.performance.now();
let i;

for (i = 0; i < 100000; i++) {
  let a = [0, 1, 2];
  const b = [3, 4, 5];
  
  a = a.concat(b);
}

console.log(`a=a.concat(b): ${window.performance.now() - from}`);

from = window.performance.now();

for (i = 0; i < 100000; i++) {
  const a = [0, 1, 2];
  const b = [3, 4, 5];
  
  a.push(...b);
}

console.log(`a.push(...b): ${window.performance.now() - from}`);