File transfer speed is very low. WinSCP does not utilize all available bandwidth. How do I improve the transfer speed?

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).

In case the speed is throttled by CPU, it may help if you choose different encryption algorithm on Login dialog (supposing you are using SSH-based file transfer protocol, such as SFTP or SCP). When using SSH, file transfers in WinSCP are encrypted and encryption is CPU intensive. Either your local workstation or your server might not be able to encrypt file transfer stream at the same speed, your connection is able to transfer it. Blowfish is usually a lot faster than AES. Also SSH-1 is in general less CPU intensive than SSH-2. It may also help if you turn off compression, if you have turned it on before.

In case the speed is throttled by connection latency, it may help if you use SCP protocol instead of SFTP. SCP is less affected by latency. In this case, it may help if you turn on compression.

Also there’s a lots to improve in performance of WinSCP itself. So it may get better in future versions. Particularly planned integration of HPN-SSH 17 from PuTTY may improve transfer rate. 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.