Slow Speeds with WinSCP sending to Amazon S3

We are experiencing speeds of 1kb while trying to transfer to Amazon S3 bucket.

What could be the issue?

Can you transfer the files faster using any other S3 client?
We made some improvements recently:
https://winscp.net/tracker/1732
Though I'm not sure it's related to your very slow speed.
Anyway, if you send me an email, I send you back a development version of WinSCP for testing. Please include a link back to this topic in your email. Also note in this topic that you have sent the email. Thanks.