gistfile1.txt

await db.doc('foo/bar').set({});
const start = Date.now();
const promises = Array.from({length: 500}).map((_, index) => {
  db.doc("a/" + index).set({});
});
const middle = Date.now();
await Promise.all(promises);
const end = Date.now();
console.log("Midpoint " + (middle - start));
console.log("End " + (end - start));

versus:

await db.doc('foo/bar').set({});
const start = Date.now();
const promises = Array.from({length: 500}).map((_, index) => {
  db.doc("a/b/c/d/e/f/g/h/i/j/k/l/m/n/o/p/q/r/s/t/u/v/w/x/y/" + index).set({});
});
const middle = Date.now();
await Promise.all(promises);
const end = Date.now();
console.log("Midpoint " + (middle - start));
console.log("End " + (end - start));