checkpoints taking much longer than expected

Hello,

I setup a new 3-node cluster with the following specifications:

2x Intel(R) Xeon(R) CPU E5-2630 v4 @ 2.20GHz (2*20 cores)
128 GB RAM
8x Crucial MX500 1TB SSD's

FS is ZFS, the dataset with the PGDATA directory on it has the following
properties (only non-default listed):

NAME PROPERTY VALUE SOURCE
tank/pgsql used 234G -
tank/pgsql available 3.28T -
tank/pgsql referenced 234G -
tank/pgsql compressratio 2.68x -
tank/pgsql compression lz4 inherited from tank
tank/pgsql atime off inherited from tank
tank/pgsql canmount on local
tank/pgsql xattr sa inherited from tank

My postgresql.conf (only changed from default listed):

hba_file = '/var/lib/pgsql/pg_hba.conf'
listen_addresses = '*'
max_connections = 800
shared_buffers = 8GB
work_mem = 64MB
maintenance_work_mem = 2GB
autovacuum_work_mem = 1GB
dynamic_shared_memory_type = posix
effective_io_concurrency = 200
max_worker_processes = 50
max_parallel_maintenance_workers = 8
max_parallel_workers_per_gather = 8
max_parallel_workers = 40
wal_level = replica
synchronous_commit = off
full_page_writes = on
wal_log_hints = on
wal_buffers = 128MB
checkpoint_timeout = 60min
max_wal_size = 8GB
min_wal_size = 1GB
checkpoint_completion_target = 0.9
archive_mode = on
archive_command = 'pgbackrest --stanza=pgdb2 archive-push %p'
max_wal_senders = 10
wal_keep_segments = 20
hot_standby = on
hot_standby_feedback = on
random_page_cost = 1.5
effective_cache_size = 48GB
default_statistics_target = 500 # range 1-10000
idle_in_transaction_session_timeout = 30min # in milliseconds, 0 is disabled
shared_preload_libraries = 'timescaledb, pg_cron'
max_locks_per_transaction = 512
timescaledb.max_background_workers = 8

My problem is that checkpoints are taking a long time. Even when I run a
few manual checkpoints one after the other, they keep taking very long, up
to 10 minutes:

2019-06-14 15:21:10.351 CEST [23657] LOG: checkpoint starting: immediate
force wait
2019-06-14 15:25:57.655 CEST [23657] LOG: checkpoint complete: wrote
139831 buffers (13.3%); 0 WAL file(s) added, 148 removed, 40 recycled;
write=284.751 s, sync=0.532 s, total=287.304 s; sync files=537,
longest=0.010 s, average=0.000 s; distance=2745065 kB, estimate=2841407 kB
2019-06-14 15:26:01.988 CEST [23657] LOG: checkpoint starting: immediate
force wait
2019-06-14 15:30:30.430 CEST [23657] LOG: checkpoint complete: wrote
238322 buffers (22.7%); 0 WAL file(s) added, 0 removed, 172 recycled;
write=264.794 s, sync=0.415 s, total=268.441 s; sync files=378,
longest=0.011 s, average=0.001 s; distance=2834018 kB, estimate=2840668 kB
2019-06-14 15:30:44.097 CEST [23657] LOG: checkpoint starting: immediate
force wait
2019-06-14 15:37:01.438 CEST [23657] LOG: checkpoint complete: wrote
132286 buffers (12.6%); 0 WAL file(s) added, 54 removed, 96 recycled;
write=366.614 s, sync=2.975 s, total=377.341 s; sync files=467,
longest=0.095 s, average=0.006 s; distance=2444291 kB, estimate=2801030 kB

What is going on? It doesn't seem like normal behaviour?