Update in trigger - Mailing list pgsql-sql

Hi,
I have a problem with trigger in 7.1.3.
I have a table with after insert trigger, which is making updates in
another table.
I recognized that the more records I have in first table the slower is
my insert.
If my trigger is doing something else, for example inserts in another
table, it becomes * 20 times * quicker, and the speed of my insert
doesn't depend on tablesize.
Don't I have to make updates in trigger?

Thanks in advance for any help.
Lajos