Performance with a very large queue about pq HOT 9 CLOSED

thematrix=> explain analyze SELECT * FROM thematrix.queue WHERE q_name = 'matrix' AND dequeued_at IS NULL ORDER BY schedule_at nulls first, expected_at nulls last, id LIMIT 1;
                                                                         QUERY PLAN                                                                         
------------------------------------------------------------------------------------------------------------------------------------------------------------
 Limit  (cost=221449.70..221449.70 rows=1 width=646) (actual time=560.917..560.918 rows=1 loops=1)
   ->  Sort  (cost=221449.70..222840.64 rows=556377 width=646) (actual time=560.917..560.917 rows=1 loops=1)
         Sort Key: schedule_at NULLS FIRST, expected_at, id
         Sort Method: top-N heapsort  Memory: 26kB
         ->  Index Scan using priority_idx_no_queue on queue  (cost=0.42..218667.81 rows=556377 width=646) (actual time=0.182..388.776 rows=595240 loops=1)
               Index Cond: (q_name = 'matrix'::text)
 Planning time: 0.179 ms
 Execution time: 560.955 ms
(8 rows)

                                   Table "thematrix.queue"
   Column    |           Type           |                     Modifiers                      
-------------+--------------------------+----------------------------------------------------
 id          | bigint                   | not null default nextval('queue_id_seq'::regclass)
 enqueued_at | timestamp with time zone | not null default now()
 dequeued_at | timestamp with time zone | 
 expected_at | timestamp with time zone | 
 schedule_at | timestamp with time zone | 
 q_name      | text                     | not null
 data        | json                     | not null
Indexes:
    "queue_pkey" PRIMARY KEY, btree (id)
    "priority_idx_matrix" btree (schedule_at NULLS FIRST, expected_at NULLS FIRST, q_name) WHERE dequeued_at IS NULL AND q_name = 'matrix'::text
    "priority_idx_no_queue" btree (schedule_at NULLS FIRST, expected_at NULLS FIRST, q_name) WHERE dequeued_at IS NULL AND q_name <> 'queue'::text
    "priority_idx_queue" btree (schedule_at NULLS FIRST, expected_at NULLS FIRST, q_name) WHERE dequeued_at IS NULL AND q_name = 'queue'::text
Check constraints:
    "queue_q_name_check" CHECK (length(q_name) > 0)
Triggers:
    pq_insert AFTER INSERT ON queue FOR EACH ROW EXECUTE PROCEDURE pq_notify()