solution

BEGIN;
select max(moment) from authors where interesting;
-- Planning Time: 0.294 ms
-- Execution Time: 0.655 ms
create temporary table actual_users as select user_id from authors where moment > '2020-08-21 01:43:20.816825';
-- SELECT 154265
-- Planning Time: 0.189 ms
-- Execution Time: 144.185 ms
create temporary table actual_videos as select  id, author, loaded from videos where author in (select user_id from actual_users);
-- SELECT 2395342
-- Planning Time: 0.361 ms
-- Execution Time: 238069.057 ms
update authors set interesting = 't' where user_id in (select author from actual_videos where exists (select null from videos v where v.loaded is not null and v.author = author) group by author having count(*) > 50);
-- UPDATE 7490
-- Planning Time: 0.669 ms
-- Execution Time: 3868.271 ms
COMMIT;
-- Всего около четырех минут