MERGE: performance advices

I need to merge 2 tables:

update d set c1=s.c1, c2=s.c2... from s where d.pk=s.pk;
insert into d (pk, c1, c2, ...) select pk, c1, c2, c3 from s
  where s.pk not in (select pk from d);

Any strategy to make it faster? Including modifying postgres.conf
temporary?

Considering I've no concurrency problems. The tables I'm dealing
with are "read only" for everything else other than the merge
process.

thanks

--
Ivan Sergio Borgonovo
http://www.webthatworks.it