Bad performace of a query

I have this query:

SELECT DISTINCT isbn, CURRENT_TIMESTAMP, 1
              FROM librosdisponibilidadtemp
             WHERE proceso = ai_proceso
               AND gen_isbn_pais(isbn) IN (SELECT pais FROM raizpaises)
               AND NOT EXISTS
                   ( SELECT isbn
                       FROM libros
                      WHERE isbn = librosdisponibilidadtemp.isbn)
               AND NOT EXISTS
                   ( SELECT isbn
                       FROM isbns_a_descubrir
                      WHERE isbn = librosdisponibilidadtemp.isbn);

and the plan execution is
Unique  (cost=133558107.45..133558128.13 rows=414 width=21) (actual time= 790552.899..790553.098 rows=9 loops=1)
  ->  Sort  (cost=133558107.45..133558112.62 rows=2068 width=21) (actual time=790552.882..790552.944 rows=9 loops=1)
        Sort Key: isbn, now(), 1
        ->  Index Scan using librosdisponibilidadtemp_idx_proceso on librosdisponibilidadtemp  (cost= 1.01..133557993.56 rows=2068 width=21) (actual time=5722.607..790552.588 rows=9 loops=1)
              Index Cond: (proceso = 28465)
              Filter: ((hashed subplan) AND (NOT (subplan)) AND (NOT (subplan)))
              SubPlan
                ->  Seq Scan on isbns_a_descubrir  (cost=0.00..8067.91 rows=1 width=21) (actual time=30.044..30.044 rows=1 loops=2025)
                      Filter: ((isbn)::bpchar = $1)
                ->  Index Scan using "libros_idx_ISBN" on libros  (cost=0.00..5.95 rows=1 width=21) (actual time=12.938..12.938 rows=1 loops=50512)
                      Index Cond: (isbn = $1)
                ->  Seq Scan on raizpaises  (cost= 0.00..1.01 rows=1 width=10) (actual time=0.764..0.871 rows=1 loops=1)
Total runtime: 790553.561 ms

The libros table has 1200000 regs.
The isbns_a_descubrir table has 300000 regs.
The librosdisponibilidadtemp table has 50000 regs.

does anybody can explain me, why using index ibrosdisponibilidadtemp_idx_proceso is so slow and the others conditions are good enough
Thanks everybody