Resilvering abnormal slow

[ak1]

Resilvering lasts more than 16 days on 2 Systems with equivalent hardware.
According to zpool status the speed (scanned & issued) drops down to 40-50MB/s.
This is not normal since resilverings in different Supermicro Chassis
(with the same model/firmware of HBA & HDDs) finish in a much more shorter time.
There are no errors in the logs. HDDs from Toshiba are CMR (no SMR!)

Any ideas?
I am grateful for any help


TrueNAS-12.0-U6

Hardware
Chassis: Supermicro CSE-836BE1C-R1K03B with SAS3 single-expander backplane BPN-SAS3-836EL1 and
1000W Power Supply (200-240V) Redundant, 80+ Titanium

Motherboard: Supermicro X11SPi-TF
CPU: Intel Xeon LGA3647 Silver 4215R 3.2GHz 8-Core
RAM: 256GB (4x64GB Samsung DDR4 2933 ECC Reg.)

Hard drives:
Boot Drives (Mirror)
2x Kioxia HK6-R 480GB SSD Enterprise 2.5" SATA TLC 3D NAND DWPD

Pool drives:
16x Toshiba MG06SCA10TE 10TB 3.5" SAS-3 7.2K 512e 256MB Cache
Firmware: MG06SCA_0107.ftd
3x vdevs, each with 5 drives, RAIDZ1
1x Hotspare
Used space according to GUI is about 50%

Hard disk controller:
1x HBA Broadcom - LSI SAS 9400-8i 12Gbit/s LSISAS3408 Controller
Firmware: Version 20.00 (HBA_9400-8i_SAS_SATA_Profile.bin)