Android Question FirebaseNotifications -- delays up to 20 seconds

We're using FCM currently to send user updates.
However sometimes we notice a delay of the return of the FCM-server. This delays is sometimes up to 20 seconds.
This also happens if sending the same message twice.

See below, sending the message at 09:33:49 but our logging (server-side) writes the return value (iOk=1) at 09:34:09.

19-10-2020 09:34:09.316 PDA=1227 iOk=1 {"data":{"type":"2","param":"19-10-2020 09:33:49,441","notify":"1","message":""},"to":"fqscM9evazs:APA91bFp1b2dZuqLHJAf8lk_Y59KQrgTX3QQTeuCXDG..."}

When sending to FCM where not using ttl.

Any thoughts what this could be?