after I tried out early omv4 on my backup server but went back to omv3 last year, I gave omv4 another chance now. On same hardware as before - see my signature -, just on a different drive, so that I simply can swap back to running omv3.
Now I have the following: With one of my filesets for testing, some 3.3GB of mp3s in 25 folders, which are blown by SMB to gb ethernet at full link speed, omv4 writes with 85-95MBps and collapses down to <1MBps inbetween, whereas omv3 writes with some 10MBps more and breaks down to about 30MBps only. (AFAIS the break-down is because the fileset is 2 main folders, with 22 subfolders in one of them.) While I tested, there were definitlvely no other accesses to omv to steel performance.
Although the hardware, I found this out with, is my private backaup server, and for this purpose, speed is not the most important feature, it's my testing system, so I'm still interested in knowing the reason of this. And what I can do to overcome it, e.g. use an i5 rather than the i3, and/or install more RAM.