[Samba] Performance issues since upgrading to 3.X to 4.X

[Dragoniade]

Hello,
I'm a bit desperate so I'm trying to seek help here.

Many year ago, I built myself 2 file servers (5 disks enclosure for 
data, 2 disks enclosure for the OS) on a computer.
It was running on Debian 7.11 for years without an issue and I was 
getting 75MB/95MB when transferring file from it to a Windows client, 
all having a 1gbs network card.
https://i.ibb.co/N618Z4W/Debian-7.png

However, after upgrading to Debian 8 and more recently Debian 11, I keep 
noticing a  huge performance drop.
The near 100 MB drop to a roller coaster of between 25MB and 60 MB/S, 
the average being 40 MB/S


I've been googling trying various 'performance' tweak', but nothing seem 
to work as expected.
https://i.ibb.co/RPjbMKV/Debian-10-Tweaks.png
https://i.ibb.co/mqwZtgR/Debian-10-No-Tweaks.png

The latest bloc of 'performance' setting were those:
# Settings to enhance performance:
strict locking = no
read raw = yes
write raw = yes
oplocks = yes
max xmit = 65535
deadtime = 15
getwd cache = yes
socket options = TCP_NODELAY SO_SNDBUF=128480 SO_RCVBUF=128480
max protocol = SMB2
# End of performance section


I've also tried an empty smb.conf file to fallback to default. Nothing 
seem to work.

Spec of the current machine:
CPU: Intel(R) Core(TM)2 CPU 6600 @ 2.40GHz
RAM: 2 GB
Ethernet controller: Qualcomm Atheros AR8121/AR8113/AR8114 Gigabit or 
Fast Ethernet (rev b0)
SATA controller: Marvell Technology Group Ltd. Device 9215 (rev 11)

I know this isn't my machine though, as I can rsync at the same speed on 
both version of the OS.
Debian 7
sent 7,962 bytes received 4,467,150,861 bytes 81,966,216.94 bytes/sec
total size is 4,466,564,278 speedup is 1.00

Debian 11
sent 8,574 bytes received 4,467,697,711 bytes 80,499,212.34 bytes/sec
total size is 4,466,564,278 speedup is 1.00


Anyone have any idea why such a speed drop? What default config could be 
triggering this drop?
All point out to the upgrade from Samba 3 to Samba 4, the lastest being 
tested being 4.9.5-Debian
I'm only interested in the File sharing of Samba. I have no need for any 
of the Active Directory addition added in Samba 4 that may have 
introduced this performance hit.

Any help on what to do to fix this would be greatly appreciated.

Thanks