Streaming Replication delay getting bigger

Hi guys,

I've got some database servers in USA (own data center) and also @ AWS
Japan.

*USA:*
master01
slave01 (Streaming Replication from master01 + wal_files)
slave02 (Streaming Replication from master01 + wal_files)

*Japan: (Cascading replication)*
slave03 (Streaming Replication from slave02 + wal_files)
slave04 (Streaming Replication from slave02)

*Running this query on slave02:*

select now() - pg_last_xact_replay_timestamp() AS replication_delay;
replication_delay
-------------------
00:00:00.802012
(1 row)

*Same query on slave03 and slave04:*

select now() - pg_last_xact_replay_timestamp() AS replication_delay;
replication_delay
-------------------
00:56:53.639516
(1 row)

*slave02:*

SELECT client_hostname , client_addr ,
pg_xlog_location_diff(pg_stat_replication.sent_location,
pg_stat_replication.replay_location) AS byte_lag FROM pg_stat_replication;
client_hostname | client_addr | byte_lag
-----------------+---------------+----------

| slave03 | 2097400

| slave04 | 3803888

(2 rows)

Why is that delay that big? Is it because networking issue? I tried to find
out what the cause is, but couldn't find anything.

SCP and FTP (big files) between those servers are really fast, +1.0MB/s.
I'm using PostgreSQL 9.2.14

Thanks!
Patrick.