Can instant notifications be enabled without having FCM?

Retek

I don't have FCM enabled, and in some apps the notifications arrive slow [eg signal sometimes doesn't send notifications for messages to me until hours after they've actually been sent, or until Ive reopened the app ]

Or is FCM required if you want instant notifications?

[deleted]

Retek make sure that Signal or similar runs with unrestricted battery

App info > App battery usage > Unrestricted

Do NOT use battery saver.

That of course applies only to the apps that provide their own backend for notifications, independent of Google Firebase.

DeletedUser28

Retek Or is FCM required if you want instant notifications?

Depends on the app. Many, like Signal or WhatsApp or anything from F-Droid, work without FCM. Others need it.