An Inesrt on a partitioned table become very slow

Hey,

I use Postgres 10.1, and I have the following structure:
Parent table with columns:
range_value: double, timestamp: timestamp, device_id: integer, count:integer

which is partitioned with Partition key: LIST (range_value) which has a
value of 1 - 26.
So basically it has 26 child partitions.

Each one of those partitions it partitioned by range. essentially a daily
partition.

This is how the perant_table_with_range_value_1 is conofigured:

Partition of: perant_table FOR VALUES IN ('1')
Partition constraint: ((range_value IS NOT NULL) AND (range_value = ANY
(ARRAY['0'::double precision])))
Partition key: RANGE ("timestamp")

At the moment I have 128 days of partitions in each one of the ranged
partitioned.
Which means a total of 26 * 128 = 3328 total partitions.

My problem is that inserts became very slow.
I used to bulk insert 10000 rows in 6 seconds, now it takes 45 seconds.

Is this an expected behavior? is there something I missing?
Thank you!