What is 4x4x4x4

question 970 evo very slow (x570 Tomahawk)

Good morning,

Unfortunately, I am currently having a strange problem with my new build.
My installed Samsung 970 EVO is miles away from its (theoretically) achievable values.
With CrystalDisk and Samsung Magician, read / write rates of 835 Mb / s respectively. 838 Mb / s determined.

The disk is installed in the M2_2 slot of my motherboard, below the GPU (x570 Tomahawk). In theory, both slots, M2_1 and M2_2, should be able to support up to PCIe 4.0 x4.
According to HWinfo and Crystal Disk, the connection with PCIe 3.0 x4 is correct. There are no temperature problems either (~ 20-35 ° C in idle and 50 ° C under load).

The current Samsung drivers, firmware and controllers are installed. At the moment I have the current beta bios from MSI on it, as well as the latest chipset drivers.
In the BIOS itself, unfortunately, I hardly found any possibilities to adjust the PCIe settings for the drives. I can only choose whether the chipset or the processor should generally run via PCIe 2,3,4 or Auto.
For PCIe_E1 I can at least define the configuration of the lanes. I simply set that from Auto to 4x4x4x4 as a test. With that I cut my GPU, of course, but I have - at least - doubled it
Reached read and write speeds on the NVMe. It doesn't make sense to me, but at least it shows that the drive can do more ...
In the M2_1 slot I have another SSD that runs as expected.

To the system:
- 5900x
- RTX 3090
- 860 EVO (connected via SATA port 6)
- 4x8 Corsair Vengeance

I disconnected the 860 once for a test, that had no effect (but there should actually be an interaction with the board).
A clean Windows installation did not bring any improvement either.

Any ideas?

Thanks !!