Bad performing DELETE

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I have two tables: t_haus is about 1400 row and t_host has 364000 entries.
Both tables are indexed on edvnr. I did a vacuum on my db and all indices
are rebuild.
I want to delete all Entries in t_haus where a row can be found in t_host.
When using "delete from t_haus where t_haus.edvnr=t_host.edvnr; " the
database performs extremely bad.
explain delete from t_haus where t_haus.edvnr=t_host.edvnr;

NOTICE:  QUERY PLAN:

Merge Join  (cost=52178.53..56754.63 rows=6299767 width=14) ->  Sort  (cost=52038.25..52038.25 rows=364359 width=4)
 ->  Seq Scan on t_host  (cost=0.00..11700.59 rows=364359 width=4) ->  Sort  (cost=140.27..140.27 rows=1729 width=10)
   ->  Seq Scan on t_haus  (cost=0.00..47.29 rows=1729 width=10)
 

EXPLAIN

What can I do to speed it up?
I've also had troubles with DELETE before (when doing joins in the
WHERE clause).
   Hans