Why has open-iscsi two times slower writes than Samba via 10G Ethernet?

First of all, the RAID-6 is the problem because of the double parity calculation. Secondly, you can connect iSCSI target twice in MS iSCSI Initiator, enable RR or Least Queue Depth (unfortunately, Win10 doesn't support multipathing, so you can test it with Windows Server instead).

In fact, block level access must be faster than file level access. What kind of benchmarking tool you are using from the Windows site? I would recommend using diskspd or FIO. Additionally, you can use something like Starwind as a much faster iSCSI target.

https://www.starwindsoftware.com/starwind-virtual-san#Hyper-V

iSCSI should be used on block level, your setup description sounds like you are using a file-system, placing a file on it and then running this file as iSCSI block layer.

This is far from being ideal, and definiteley not a setup for comparing speeds. Try using lvm on top of the raid6 for segmenting the space and staying on block layer for iSCSI, or use raid6 directly as iSCSI device.

In your current setup, data is transferred through the network, hitting a file in the filesystem, which is (most likely) not optimized for this type of workload, and as well shared with other processes. It is possible to do such setup with iSCSI, but should be considered as unoptimized fallback solution.