And I'm not talking about decision, it can take many months and sometimes even years. I'm talking about application receipt notice. It can take 4-8 weeks to get just that in the mail. So if you're only in the US for 21 days, you can miss it even if you file I-131 same day as you arrive.
Immigration in the US is a slow, expensive and nerve racking process. It's typically not fast or efficient. Some folks get lucky with quick turnaround, but it's the minority.
I think you already know this, since you went through immigrant visa process and it wasn't fast?