Why is USB 3.0 slow on my dual boot Ubuntu system?

Why is USB 3.0 slow on my dual boot Ubuntu system?

On my computer, which dual-boots Ubuntu LTS (latest) and Windows 7, the
USB 3.0 speed is very fast on Windows but very slow on Ubuntu. Anybody
know why, or how it can be fixed?
Here are the speeds using exactly the same hardware:
Ubuntu LTS: Less than 5 MB/sec, estimated using /bin/du while copying files.
Windows 7: 168 MB/second as reported by Windows
The source drive, an external USB 3.0 drive, is formatted with NTFS. The
destination drive is an internal SSD formatted with ext4 (in the Linux
case) or NTFS (in the Windows case).
The cable and external drive are definitely both USB3. This computer
itself has a very fast 6-core CPU and 32 GB RAM, so computer power is not
an issue. It's a brand new computer so it's possible something is
configured wrong.
Thank you.