The first user of the morning waits eight seconds for a reply. Everyone after them waits under a second. Your serverless platform shut the instance down overnight and had to build a new one, which is a cold start.
The old fix was a timer-triggered function that pinged your own app every few minutes. You do not need to write that any more. Every major platform now has a setting for it.
Note: Choosing Node.js or Python over Java or .NET used to be standard cold start advice. Interpreted runtimes still initialise faster, but SnapStart narrows the gap enough that runtime choice should no longer decide your language.
Formerly Cloud Functions, and the same settings apply to Cloud Functions for Firebase.
Always-warm instances bill while idle, so match the setting to real traffic rather than turning it on everywhere.