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..8067.91rows=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