Upuah
Outgoing WhatsApp messages leave with a few seconds between them, and there is an hourly and a daily ceiling on each number.
This is deliberate. WhatsApp bans numbers that send in bursts, and a banned number cannot be recovered. A brand-new number gets a lower ceiling for its first weeks and it climbs on its own as the number ages.
A reply that hits the ceiling is not dropped. It waits in the queue and goes out as soon as there is room, which is usually seconds later.
If you send a lot every day, connect a second number rather than pushing one number harder.