bad COPY performance with NOTIFY in a trigger

Hi.

A table has a trigger.
The trigger sends a NOTIFY.

Test with COPY FROM shows non-linear correlation between number of inserted
rows and COPY duration.

Table "test.tab"
Column | Type | Modifiers
---------+---------+-------------------------------------------------------
id | integer | not null default nextval('test.tab_id_seq'::regclass)
payload | text |
Indexes:
"tab_pkey" PRIMARY KEY, btree (id)
Triggers:
trg AFTER INSERT ON test.tab FOR EACH ROW EXECUTE PROCEDURE test.fun()

Test Series 1. Trigger code:
BEGIN RETURN NULL; END
You can see linear scaling.

rows=40000
191ms COPY test.tab FROM '/tmp/test.dat'
201ms COPY test.tab FROM '/tmp/test.dat'
rows=80000
426ms COPY test.tab FROM '/tmp/test.dat'
415ms COPY test.tab FROM '/tmp/test.dat'
rows=120000
634ms COPY test.tab FROM '/tmp/test.dat'
616ms COPY test.tab FROM '/tmp/test.dat'
rows=160000
843ms COPY test.tab FROM '/tmp/test.dat'
834ms COPY test.tab FROM '/tmp/test.dat'
rows=200000
1101ms COPY test.tab FROM '/tmp/test.dat'
1094ms COPY test.tab FROM '/tmp/test.dat'

Test Series 2. Trigger code:
BEGIN PERFORM pg_notify('test',NEW.id::text); RETURN NULL;
You can see non-linear scaling.

rows=40000
9767ms COPY test.tab FROM '/tmp/test.dat'
8901ms COPY test.tab FROM '/tmp/test.dat'
rows=80000
37409ms COPY test.tab FROM '/tmp/test.dat'
38015ms COPY test.tab FROM '/tmp/test.dat'
rows=120000
90227ms COPY test.tab FROM '/tmp/test.dat'
87838ms COPY test.tab FROM '/tmp/test.dat'
rows=160000
160080ms COPY test.tab FROM '/tmp/test.dat'
159801ms COPY test.tab FROM '/tmp/test.dat'
rows=200000
247330ms COPY test.tab FROM '/tmp/test.dat'
251191ms COPY test.tab FROM '/tmp/test.dat'

O(N^2) ????