flatten an array of objects with a property that contains an array

speed test

const arr = [
  { sub: [ 1001, 1002, 1003, 1004, 1005 ] },
  { sub: [ 2001, 2002, 2003, 2004, 2005 ] },
  { sub: [ 3001, 3002, 3003, 3004, 3005 ] },
  { sub: [ 4001, 4002, 4003, 4004, 4005 ] },
  { sub: [ 5001, 5002, 5003, 5004, 5005 ] },
];

const startDates = [];
const endDates = [];
const commands = [];
results = [];

const cycles = 1000000;

console.log( 'running [].concat( ...arr.map( x => x.sub ))', cycles, 'times' );
commands.push( '[].concat( ...arr.map( x => x.sub ))' );
startDates.push( new Date().getTime() );
for( let index = 0; index < cycles; index++ ) {
  [].concat( ...arr.map( x => x.sub ));
}
endDates.push( new Date().getTime() );

console.log( 'running arr.forEach( x => x.sub.forEach( y => res.push( y ) ) )', cycles, 'times' );
commands.push( 'arr.forEach( x => x.sub.forEach( y => res.push( y ) ) )' );
startDates.push( new Date().getTime() );
for( let index = 0; index < cycles; index++ ) {
  let res = [];
  arr.forEach( x => x.sub.forEach( y => res.push( y ) ) );
}
endDates.push( new Date().getTime() );

for (let index = 0; index < startDates.length; index++) {
  console.log( cycles + 'x   ' + (endDates[index] - startDates[index]) + 'ms       ' + commands[index] );
}

1000000x   407ms       [].concat( ...arr.map( x => x.sub ))
1000000x   158ms       arr.forEach( x => x.sub.forEach( y => res.push( y ) ) )