SOLVED!
i've spent another day trying to understand what's going on. google didn't help me at all.
suddently i've noticed this:
isdct tool has a "set" parameter "physpeed"
why not to try. so i typed:
isdct set -intelssd 0 physpeed=6
and after restart - voila!
and
finally i got the speed as it should be. still under 500MB/s (maybe it's normal?), but at least way better than before, at 3.0 Gb/s connection.
here's another hdtune benchmark. win10 never stops "doing something", i see drive action all the time in task manager, so i guess that's why benchmark shows lower speeds than at linux.
so the problem was in that kind of internal setting of this particular ssd.
i have no clue why it was set to 3.0 Gb/s, but it's good that the truth has been found.
will do some more tests tomorrow, but guess the drive is now working at full speed.