This new feature works great! Our workaround had been to manually split the large file with 7zip, upload it with the old version of WinSCP as multiple concurrent transfers, then manually re-assemble on the other end. This upcoming version of WinSCP does that for us automatically, with a single drag-n-drop of one file.
In terms of usability, I was initially caught out by Enabling only "Preferences | Transfer | Background -> "Use multiple connections for single transfer". This gave me just our slow single connection speed.
Then I came back to that Preferences screen and enabled the child preference: "Use multiple connections for single files above:" and went with the default of 100 MB. With both of these settings enabled together, I was able to multiple connections happening on automatically split parts.
Great UI display by the way, it's very clear what it is happening when a multi-part single file transfer is happening. At "Maximal number of transfer at the same time" set to 2, I saw roughly a doubling in our overall transfer rate for the large file. When I increased to 4 concurrent transfers (of 4 file parts), I saw roughly a 4-fold speed increase.
Attached is GB-sized example screenshot, but our real files are TB-sized. Great stuff!
Thanks,
Guy