Why does the Samsung EVO 860 SSD in SATA 2 run at 142 Mb/s when the peak should be around 300 Mb/s?

Question

linux

Hello!
YES PC:
CPU: AMD FX 8350 (4.0 Ghz)
GPU: MSI GTX 1050 Ti GAMING X 4G
RAM: 16 Gb (1866 Mhz) HyperX (Dual: 4x4 GB)
OS: Ubuntu 18.04
MB: Asus M5A78L-M PLUS/USB3
SSD Samsung EVO 860 (250 GB) (EXT4) - FOR SYSTEM (6 months)
SSD Silicon Power (60 GB) (NTFS) - FOR WIN 10 (4 years)
THERE IS A PROBLEM:
SSD Samsung EVO 860 working in SATA 2 gives out a speed of 142 MB /with.
SSD Samsung EVO 860 working in SATA 3 gives a speed of 549 Mb / s.
An old SSD is installed next to it, which, working in SATA 2, produces 280 Mb / s.
Shouldn't the Samsung EVO 860 SSD in SATA 2 deliver speeds under 300 Mb / s?
OR FROM SCREENSHOTS:
1. OLD SSD (SATA 2)

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2. NEW SSD (SATA 2)

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3. NEW SSD (SATA 3)

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DOES ANYONE HAVE A SUGGESTION WHY THE Samsung EVO 860 SSD IN SATA 2 WORKS NOT AT A PEAK OF 300 MB / S, BUT 2 TIMES LOWER?

Answer 1

The problem was in UBUNTU 18.04 OS, the problem is software, not mechanical.
The hardware is the same, without changing anything inside the PC, I just changed Ubuntu to Windows 10 and there are no speed problems. Comments a la "change the cable, change the port, the problem is in the hardware, etc." - could not help. Thanks anyway for the ideas and your time.
I have been using Windows 10 all day long, filled it up with the same working software as in Ubuntu + loaded it.
The system is very responsive, I wish Ubuntu could run at the same speed.