Promediando una lista de TIMESTAMP(6) WITH TIME ZONE veces

Usa el AVG función

SELECT avg(cast(lastprocesseddate as date)-cast(importeddate as date))
FROM feedqueueitems 
WHERE eventid = 2213283 
ORDER BY written DESC;

En la base de datos con la zona horaria +1 para la fecha importada y la última fecha procesada es UTC

SELECT avg(cast(cast(lastprocesseddate as timestamp with time zone) at time zone '+01:00' as date)-cast(importeddate as date))
FROM feedqueueitems 
WHERE eventid = 2213283 
ORDER BY written DESC;