And since you deny the ignore comment - this forum has complaints about slow FTP dating back to 2007.
What would you call it?
OK, so seems I missed a few release notes with some mention of FTP.. nothing to address the speed issue - but the priority still seems broken.
We are going to completely replace the SFTP code, but that's not a small task and not one we're going to start until finishing other work.
The main driver for this isn't upload speed (although that definitely has become a problem for more people over the years as internet speeds get faster, while there seems to be a bottleneck due to latency in the code we're using), but the fact it's become difficult to update the code the existing SFTP component is based on. Choosing a component with better performance will be part of that process.
If you don't believe me then I don't know what else to say really.
Since it's not a high priority to fix this issue, and since it's a now well known and document issue, you should at least put a disclaimer on the add-on purchase page or stop selling it under the guise of being usable for sftp uploads.
There's a free trial so people can already test it works for them before buying.
Your implication that it's the users' fault for not finding out the add-on is broken because they didn't try the free trial is embarrassing.
The cherry on top is that you don't offer refunds for the broken add-on.