Later builds of Windows 10 have improved on the TCP implementation in general, adding the CUBIC congestion control algorithm, etc.
You may want to try changing it back to CTCP, with or without ECN to see if it makes a difference.
Does the upload speed vary throughout the day, or is it always throttled to about the same?
Make sure there is no third-party anti-virus/anti-malware/security software installed and running in the background, as many tend to try and inspect every packet, effectively throttling the connection.
You can also try resetting TCP/IP and Windsock, there is an option in the "File" menu of the TCP Optimizer.
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