JSFiddle - React, Tailwind, and code Playground

JavaScript

let dataMap = new Map();
const diff = 1000000;
let key = null;

console.time();
for(let i = 0; i < 1000000; i++){
	key = `key${i}`;
	dataMap.set(key, [{'id': i, 'name': `obj${i}`}]);
	dataMap.get(key).push({'id': i + diff, 'name': `obj${i + diff}`})
}   
console.timeEnd();

dataMap = new Map();

console.time();
for(let i = 0; i < 1000000; i++){
	key = `key${i}`;
	dataMap.set(key, {'id':[i], 'name': [`obj${i}`]});
	dataMap.get(key).id.push(i + diff);
	dataMap.get(key).name.push(`obj${i + diff}`);
}
console.timeEnd();