Почему psycopg2 такой медленный?

# DB Version: 11
# OS Type: linux
# DB Type: oltp
# Total Memory (RAM): 32 GB
# CPUs num: 8
# Connections num: 50
# Data Storage: ssd

max_connections = 50
shared_buffers = 8GB
effective_cache_size = 24GB
maintenance_work_mem = 2GB
checkpoint_completion_target = 0.9
wal_buffers = 16MB
default_statistics_target = 100
random_page_cost = 1.1
effective_io_concurrency = 200
work_mem = 41943kB
min_wal_size = 2GB
max_wal_size = 4GB
max_worker_processes = 8
max_parallel_workers_per_gather = 4
max_parallel_workers = 8

import psycopg2
conn = psycopg2.connect(dbname='data', user='postgres', password='...')
cur=conn.cursor()
cur.execute('CREATE TABLE IF NOT EXISTS users (user_id serial PRIMARY KEY, username VARCHAR(50) NOT NULL, email VARCHAR(355) UNIQUE NOT NULL)')			
conn.commit()
cur.execute('DELETE FROM users')
conn.commit()
for i in range(50000):
    cur.execute('insert into users (username, email) values (%(username)s, %(email)s)',
      {"username": 'balabldasblablabladblbalablablablablablabl{}'.format(i), "email": 'mialmiaolmialimalamalimailm@mailamalmalaiimliamilalmialmialim{}.com'.format(i) }
    )
conn.commit()

import timeit
def readrows(cur, number):
	cur.execute("SELECT username FROM users LIMIT {}".format(number))
	for row in cur:
		pass

for count in range(10000, 60000, 20000):
  print('readrows(cur, {}): {}'.format(
    count,
    timeit.timeit('readrows(cur, {})'.format(count), "from __main__ import cur, readrows", number=1)
    ) )

readrows(cur, 10000): 0.00844403600000021
readrows(cur, 30000): 0.018557535000000236
readrows(cur, 50000): 0.026785620000000065
>>> psycopg2.__version__
'2.8.3 (dt dec pq3 ext lo64)'
>>> conn.server_version
100010

<A>
cur.execute("SELECT name FROM users LIMIT 10000")
<B>
for row in cur:
    pass
<C>