I checked that and this is either off or set Normal. Both have no effect on speed.

This is most probably not the WinSCP problem as i have same speed with FileZilla and even Globus. At the late night by the way i get single file speed almost as fast as ISP allows. The problem is that somebody between me and final point switched off the option allowing single file multistream downloads.

First, make sure you didn't enable debug logging:
https://winscp.net/eng/docs/faq_slow#logging

If that's not the case, did you test any other client using the same protocol?

Abruptly for more than a month my download speed with supercomputer site dropped 10 times. If i download 10 files simultaneously the total speed (while each will be slow at 1/10 of maximum possible speed) gives me OK total speed. But downloading just the single file always goes now at 1/10 maximum possible speed.

The network seems is OK, the internet service provider does not find anything wrong. The Speedtest shows normal speed, other downloads from other sites are as fast as usual. The people on supercomputer side do not find anything changed or wrong, and no one else complained about speed

Why the feature of dividing the file on several streams and simultaneous streaming all of them does not work somewhere? Who controls this feature, WinSCP, end server site, ISP, intermediate network sites?