Performance problem with query

Good morning list,

the following query takes about 15 to 20 minutes for around 2 million lines in the file myfile.txt, but with 8 million
linesit takes around 5 hours and i just don't understand why there is such a huge discrepancy in performance. 

COPY public.temp_table FROM 'myfile.txt' WITH DELIMITER '\t';

INSERT INTO public.master(pid,smid, val1, val2, chr)
            SELECT pid, smid, val1, val12, CAST(split_part((SELECT chr from public.temp_table LIMIT 1), '_', 2) as
int2)        
           FROM public.temp_table;

INSERT INTO public.values(smid, pos, chr)
        SELECT DISTINCT smid, pos, CAST(split_part((SELECT chr from public.temp_table LIMIT 1), '_', 2) as int2)
        FROM public.temp_table;

I came up with this query, because i wanted to use the COPY command to load huge files into the db, but i don't want to
loadall the columns contained in the file in only one table but copy some of them into one table and some in a second
table.As i found out with logging, the data is loaded into temp_table within 15 minutes, but to transfer it from the
temp_tableto public.master took around 5 hours and from temp_table to public.values took again only something like 10
minutes.Can it be that the cast takes up so much more time than when reading and transferring 2 million lines? 

Thanks for any advice!

--
Christian Rengstl M.A.
Klinik und Poliklinik für Innere Medizin II
Kardiologie - Forschung
Universitätsklinikum Regensburg
B3 1.388
Franz-Josef-Strauss-Allee 11
93053 Regensburg
Tel.: +49-941-944-7230