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Previously, uploading files to our FTP server, which is on the same network as our application, would be super-fast and work at around 10-13 MB/s.

We had to recently upgrade the server from Windows 2003 to Windows 2012R2 server, and also updated the SFTP host VShell to the latest version.

In doing so, our uploads have dropped to around 300 KB/s, which is a dramatic drop.
The application that performs the upload has not changed.
We were using v6.3.5 and are thinking of upgrading.

Other applications, like FileZilla, connect and upload at the expected 10 MB/s speed.
Even using the Demo apps, the speed is slow.

Has anyone experienced something like this? Are there more settings we need to be setting?

Any help would be appreciated.

Unfortunately, we couldn't reproduce the issue.
For the issue investigation, please specify the exact version of your SFTP server and give us access to your server if it is possible.

I played around with the WriteBlockSize until I found an acceptable value. Using 4 or 5 MB block sizes, I can reach 10 MB/s now, but then it slows down and levels out to about 6 or 7 MB/s. For now this is decent since we're not dealing with too many huge files. It seems with the default of 32KB causes too much back and forth and slows the speed down to below 300 KB/s. But I still cannot determine what differences there are between the old an new server - besides OS version and VShell - all options between the 2 are the same as far as we can tell.

But I have another question - Is there a way to determine the maximum write block size for the server that we're connecting to? Or is it just trial and error? Is there something on the server that we can query to get the best value?

SFTP server does not provide any information about the optimal WriteBlockSize value. That is why its optimal size must be defined experimentally for each server.