LISTEN NOTIFY sometimes huge delay

Hi all,

I have a table with time series data and on this table a trigger for
notifies:

containers_notify AFTER INSERT ON containers FOR EACH ROW EXECUTE
PROCEDURE containers_notify('containers_notify_collector')

and the function does:

PERFORM pg_notify(CAST(TG_ARGV[0] AS text), row_to_json(NEW)::text);

so that another application (java) fetches every inserted row as a JSON
for further processing every half second:

...listenStatement.execute("LISTEN 'containers_notify_collector'");
...PGNotification notifications[] =
((org.postgresql.PGConnection)notifyPGCon.getUnderlyingConnection()).getNotifications();

This works as a charm but occasionally (I think with more load on the
system) the notifications are received much time (up to hours!) after
the INSERTs.
Nevertheless no notifications become lost, they are only very late! The
delay grows, seems as a queue grows, but the java process tries to fetch
the notifications fairly fast,
so there should be no queue growing..

Versions:
PostgreSQL 10.12 on x86_64-pc-linux-gnu, compiled by
x86_64-pc-linux-gnu-gcc (Gentoo 6.4.0-r1 p1.3) 6.4.0, 64-bit
JDBC 42.2.23

The commit of the application inserting the data is ok/fast. So the
insert of the data is not slowed down.
Are the notifications delivered asynchronously to the commit/trigger?

Thanks for any help,

Peter