The Problem is not from Any Side. Its the actual Internet Connection Speed(Fiber Line between Server and Client).
In My Instance, I have a 10Gbps Bare Metal Server and a 1Gbps line in My Office. But the Server Location is Germany and Workplace is in India. Even when the Server Speed is 10Gbps, I get a Maximum upload variance of around 300KB/s - 350KB/s. This is due to Connection line between Datacenter and ISP which is this only per connection. So, That applies to your situation too.
If You want a Total Max. upload speed of Server, You need to be on the Same Network or Near the Datacenter. And as you said, There isn't anything that Terminal(SSH sFTP) can upload in 30sec and FTP Upload the same in 30 min.
You can Check your Speed by Installing Speedtest CLI on your cPanel/WHM Server. Then Select the Near about ISP of your city and see yourself about the Connection speed actually offered.