Slow download speeds

I’m getting 2-3 MB/s download speeds on 100/100 connection when using RaiDrive connected to my SFTP server.
Uploading to server is fine and I almost max out my upload speed 10-11 MB/s.

If I use FileZilla everything is fine. Upload/download speed is fine.

SFTP server is on 500/500 connection.

Basically we test the performance of upload/download on Synology and QNAP server.
To find out something we missed, could you let us know two more?

  1. SFTP server, version, OS.
  2. “100/100 connection” mean?