Осредняване на списък от пъти TIMESTAMP(6) WITH TIME ZONE

Използвайте AVG функция

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

В базата данни с +1 часова зона за importeddate и lastprocesseddate е 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;