The transfer speed can be throttled by two factors (apart from bandwidth). CPU (computation power of machines on both sides) and connection latency (how long does it take for unit of data to transfer between the two machines).
When using SSH or TLS/SSL, file transfers in WinSCP are encrypted and encryption is CPU intensive. Either your local machine or your server might not be able to encrypt file transfer stream at the same speed, your connection is able to transfer it.
WinSCP as well as most (if not all) SFTP servers cannot distribute the encryption/decryption among CPU cores, so it’s actually a capacity of a single CPU core that limits the transfer speed.
Use the Windows Task manager to see, if one of the cores is utilized to its maximum during the transfer.
In case the speed is throttled by CPU, it may help if you choose different encryption algorithm on SSH page of Advanced Size Settings dialog (supposing you are using SSH-based file transfer protocol, such as SFTP or SCP). Blowfish is usually a lot faster than AES. It may also help, if you turn off compression, if you have turned it on before.
Network delay/latency affects particularly SFTP, as it is a packet oriented-protocol. When trandferring, the SFTP client (WinSCP) sends a read/write request to the SFTP server, waits for a response; and repeats, until the end of the file.
Even if your connection is fast, if the server is far away (or slow), it takes a time for the response to arrive back. If the client spends this time uselessly waiting, your transfer speed will be low.
Most SFTP clients (including WinSCP) overcome the problem by both requesting/sending a large chunk of the file in each single read/write request and by sending (queuing) multiple requests without waiting for a response to previous. For example WinSCP can request up to 32 chunks for 32 KB each at once, totaling 1 MB (these are defaults which can be altered with
SFTPUploadQueue raw session settings). But if there’s a big discrepancy between the bandwidth and the network delay, even that 1 MB can be too small to saturate the bandwidth.
See also Bandwidth-delay product on Wikipedia.
In case the speed is throttled by a connection latency, it may help if you use SCP protocol instead of SFTP. SCP is less affected by the latency. In this case, it may help if you turn on compression. Toggling Optimize connection buffer size, in either way, can help too.
An underlying TCP protocol can suffer a similar problem too. In which case it will affect all other protocols, including SCP, FTP, WebDAV or S3.
Also there’s a lots to improve in performance of WinSCP itself 164. So it may get better in future versions.
Also note that as SSH code of WinSCP is based on PuTTY, file transfers with SSH-based protocols can hardly be faster than PuTTY (PSCP/PSFTP) is. Hence there is no point asking for speed improvements, if you get the same rate with PuTTY.
If WinSCP started being slow suddenly, check if you did not enable logging on “Debug” level inadvertently.