pipe.ts

const Observable$ = of(1).pipe(
  tap(console.log),
  delayWhenLoading(isLoading = true)
);

Observable$.subscribe();

const delayWhenLoading = <T>(isLoading: boolean, delayTime: number = defaultDelayTime): OperatorFunction<T, T> => {
  return pipe(
    delayWhen(() => {
      if (isLoading) {
        return timer(delayTime);
      }
      return timer(0);
    }),
    tap(() => console.log('emit'))
  );
};

without-pipe.ts

const Observable$ = of(1)
  .tap(console.log);

delayWhenLoading(source$, isLoading = true).subscribe();

const delayWhenLoading = <T>(source$: Observable<T>, isLoading: boolean): Observable<T> => {
  return source$
    .delayWhen(() => {
      if (isLoading) {
        return timer(delayTime);
      }
      return timer(0);
    })
    .tap(() => console.log('emit'));
};