UPGRADE TO PG11 CAUSED DEGREDATION IN PERFORMANCE

Hey,
I upgraded my pg9.6 cluster to pg11.2.
As it seems after the upgrade the duration of the same flow in my
application raised from 13 minutes to 19 minutes.

The test I did :
1.reset pg_stat_statements
2.run the applicative flow
3.collect everything from pg_stat_statements

I did this test on the env before the upgrade and after the upgrade. I took
the sum of the total_time in pg_stat_statements and sumed it up.

my env settings :
60GB RAM
16CPU
regular HD

postgresql.conf settings :
max_wal_size = 2GB
min_wal_size = 1GB
wal_buffers = 16MB
checkpoint_completion_target = 0.9
checkpoint_timeout = 30min
standard_conforming_strings = off
max_locks_per_transaction = 5000
max_connections = 500
random_page_cost = 4
deadlock_timeout = 5s
shared_preload_libraries = 'pg_stat_statements'
track_activity_query_size = 32764
log_directory = 'pg_log'
enable_partitionwise_join = on
enable_partitionwise_aggregate = on
max_worker_processes = 16               # (change requires restart)
max_parallel_maintenance_workers = 8   # taken from max_parallel_workers
max_parallel_workers_per_gather = 8    # taken from max_parallel_workers
max_parallel_workers = 16
maintenance_work_mem = 333MB
work_mem = 60MB
shared_buffers = 15129MB
effective_cache_size = 30259MB

The conf file was used in 9.6 (without all the new parallel settings).
Now the same queries run in both tests because it was the same flow. I will
be happy to hear if u have any ideas without involving any queries changes..