DELETE SQL too slow.

I'm having problems with delete commands in postgres. They are too slow.
I'm trying to delete a set o 300 rows in table with 50000 rows with a
simple command like:

DELETE FROM table WHERE field1 = '4'

When I run the explain it tells me that index_scan is being used. But to
complete the delete it takes more than 30 minutes and the CPU use never
rise above 10%. The server that runs postgres is a RedHat Linux 7.3 over
a Pentium III 900 Mhz with 512 MB RAM

Here's a part of my postgresql.conf:

max_connections = 64
sort_mem = 32168
shared_buffers = 15200
fsync = false
enable_seqscan = false
wal_buffers = 10
wal_files = 10
checkpoint_segments = 20

--
Diogo de Oliveira Biazus
diogo@ikono.com.br
Ikono Sistemas e Automação
http://www.ikono.com.br