rxjs operator, make the data not emit too fast

delayThreshold.ts

import { timer } from 'rxjs/observable/timer';
import { delayWhen } from 'rxjs/operators';

export const delayThreshold = (threshold: number, start = Date.now()) =>
  delayWhen(() => {
    const delay = threshold - (Date.now() - start);
    return timer(delay > 0 ? delay : 0);
  });