Very slow inserts when using postgres_fdw + declarative partitioning

We have been trying to partition postgresql database on google cloud using
the inbuilt Postgresql declarative partitioning and `postgres_fdw` as
explained [here][1]. Each machine that we have consists of 4 CPU cores and
8GB RAM with 10GB of disk.

We are running commands to create partition as follow:

*Shard 1:*

CREATE TABLE message_1 (
id SERIAL,

m_type character varying(20),
content character varying(256) NOT NULL,
is_received boolean NOT NULL,

is_seen boolean NOT NULL,
is_active boolean NOT NULL,
created_at timestamp with time zone NOT NULL,
room_no_id integer NOT NULL,
sender_id integer NOT NULL
);

CREATE TABLE message_2 (
id SERIAL,

m_type character varying(20),
content character varying(256) NOT NULL,
is_received boolean NOT NULL,

is_seen boolean NOT NULL,
is_active boolean NOT NULL,
created_at timestamp with time zone NOT NULL,
room_no_id integer NOT NULL,
sender_id integer NOT NULL
);

*Shard 2: *

CREATE TABLE message_3 (
id SERIAL,

m_type character varying(20),
content character varying(256) NOT NULL,
is_received boolean NOT NULL,

is_seen boolean NOT NULL,
is_active boolean NOT NULL,
created_at timestamp with time zone NOT NULL,
room_no_id integer NOT NULL,
sender_id integer NOT NULL
);

CREATE TABLE message_4 (
id SERIAL,

m_type character varying(20),
content character varying(256) NOT NULL,
is_received boolean NOT NULL,

is_seen boolean NOT NULL,
is_active boolean NOT NULL,
created_at timestamp with time zone NOT NULL,
room_no_id integer NOT NULL,
sender_id integer NOT NULL
);

*Source machine: *

CREATE SERVER shard_1 FOREIGN DATA WRAPPER postgres_fdw OPTIONS (host
'shard_1_ip', dbname 'shard_1_db', port '5432');
CREATE SERVER shard_2 FOREIGN DATA WRAPPER postgres_fdw OPTIONS (host
'shard_2_ip', dbname 'shard_2_db', port '5432');

CREATE USER MAPPING for source_user SERVER shard_1 OPTIONS (user
'shard_1_user', password 'shard_1_user_password');
CREATE USER MAPPING for source_user SERVER shard_2 OPTIONS (user
'shard_2_user', password 'shard_2_user_password');

CREATE TABLE room (
id SERIAL PRIMARY KEY,
name character varying(20) NOT NULL,
created_at timestamp with time zone NOT NULL,
updated_at timestamp with time zone NOT NULL,
is_active boolean NOT NULL
);

insert into room (
name, created_at, updated_at, is_active
)
select
concat('Room_', floor(random() * 400000 + 1)::int, '_', floor(random()
* 400000 + 1)::int),
i,
i,
TRUE
from generate_series('2019-01-01 00:00:00'::timestamp, '2019-4-30
01:00:00', '5 seconds') as s(i);

CREATE TABLE message (
id SERIAL,

m_type character varying(20),
content character varying(256) NOT NULL,
is_received boolean NOT NULL,

is_seen boolean NOT NULL,
is_active boolean NOT NULL,
created_at timestamp with time zone NOT NULL,
room_no_id integer NOT NULL,
sender_id integer NOT NULL
) PARTITION BY HASH (room_no_id);

CREATE FOREIGN TABLE message_1
PARTITION OF message
FOR VALUES WITH (MODULUS 4, REMAINDER 1)
SERVER shard_1;

CREATE FOREIGN TABLE message_2
PARTITION OF message
FOR VALUES WITH (MODULUS 4, REMAINDER 2)
SERVER shard_1;

CREATE FOREIGN TABLE message_3
PARTITION OF message
FOR VALUES WITH (MODULUS 4, REMAINDER 3)
SERVER shard_2;

CREATE FOREIGN TABLE message_4
PARTITION OF message
FOR VALUES WITH (MODULUS 4, REMAINDER 0)
SERVER shard_2;

*Problem:*

The problem we are facing is that when we are trying to insert data using
following query:

insert into message (
m_type, content, is_received, is_seen, is_active, created_at,
room_no_id, sender_id
)
select
'TEXT',

CASE WHEN s.i % 2 = 0 THEN 'text 1'
ELSE 'text 2'
end,
TRUE,
TRUE,
TRUE,
dr.created_at + s.i * (interval '1 hour'),
dr.id,
CASE WHEN s.i % 2 = 0 THEN split_part(dr.name, '_', 2)::int

ELSE split_part(dr.name, '_', 3)::int
end,
from room as dr, generate_series(0, 10) as s(i);

It is taking nearly *1 hour 50 minutes* to insert around *20 million
entries*. When we are not sharding the table, it takes around *8 minutes*
to perform the same. So, that is basically *14 times slower* than without
sharding. Are we missing anything here or inserts are that slow in sharding
using this method?

Citus seems to be performing better in insert as described in this
[video][2], so it seems a little odd to me that sharding will actually
degrade the performance by this much. I can understand it migh not have as
good performance as citus but why so much low performance.

Thanks in advance!!!

[1]: https://pgdash.io/blog/postgres-11-sharding.html
[2]: https://youtu.be/g3H4nGsJsl0