Improve performance for writing

Hello

please do not consider this email as an yet another question how to speed up writing.

The situation is different:

My algorithm stores after the computation the result as tuples in a DB.
The tuples in addition to normal values (e.g. a,b) , contains sql statements that fetch values (for instance the geometry attribute) from another table (e.g. orig_table).

e.g.

INSERT INTO dest_table (
Select a,b, s.geometry,s.length from orig_table s where s.id=?
)

The number of inserts depends on the size of the result and vary from 10,000 to 1,000,000.

My question is: how can I speed up such inserts?

Only COPY statements want work, since I need additional values
Insert statements takes long time (even if using Bulk)

What do you suggest me in such a situation?

Would it be better to perform?
- first use COPY to store values in new table
- second update the new table with values from origin table

thanks for your hints / suggestions

cheers Markus