Momentum answers
What happens if you don't follow up within an hour?
The lead is still open, but your odds have dropped — by then they've often already heard from a competitor and started that conversation instead. The lead isn't dead, but a reply the next day is reaching someone whose attention has moved on, not someone actively comparing options.
This is why "we followed up eventually" and "we followed up fast" produce very different close rates on paper-identical leads — the lead itself didn't change, only how much competing attention it picked up while it waited.