No you are not right. It is really, how the SFTP protocol is coded. Current version (3.3 and below) has most simplest SFTP implementation.
Example on download: It just asks server for few kilobytes of data and waits for them. When it receives them, it writes them to local file and issues request for next few kilobytes, etc. If your connection is slow, WinSCP waits most of the time for answer, not downloading anything.
I've just improved this. Now WinSCP issues more (four) read requests and, when it receives first response, it sends new request. So most of the time, there are four requests outstanding. It makes performance much better. I have gained about 4 times better performance. But it can vary depending on CPU speed and badwidth.
If you would like to test it, download WinSCP 3.3.1 beta build 188. Let me know, what were your results.
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