XPS 8700 USB 3.0 Ports all 6 operate at USB 2.0 speeds

How are you measuring USB speeds?

Do you have both of the latest chipset drivers:

www.dell.com/.../driversdetails

www.dell.com/.../driversdetails

Or go to Intel's download site and let it scan your hardware to see if there are even newer chipsets (and Intel Graphics drivers).

240-250 Mbps imaging is based on non windows non dell supported hardware and software. Typical USB2  hard drives can be written to at rates around 25–30 MB/s, and read from at rates of 30–42 MB/s.  You are demanding faster than theoretical speed from USB3 and Hard drives Both mechanical and SSD.  According to a USB-IF chairman, "at least 10 to 15 percent of the stated peak 60 MB/s (480 Mbit/s) of Hi-Speed USB goes to overhead.   You are also equating Mega BITS per second to Mega BYTES per second which is just plain wrong.  A low-speed rate of 1.5 Mbit/s (~183kB/s) is defined by USB 1.0.

A SuperSpeed (USB 3.0) rate of 4800 Mbit/s (~572 MB/s) is not going to be had from mechanical OR solid state Drives due to overhead.  

Acronis TrueImage 2018 reports transfer speeds of 240-250Mbps (Not MB's).  The backups seem to take twice as long as they did on my VOSTRO 460 using a USB 3.0 PCIE Card.

SpeedStep does not have the right to deny someone's request for help on this forum.

I am a user on the forum.  The fact remains that there is no support for Acronis Software on the Dell forum.  Dell does not guarantee speed or functionality of any 3rd party software. 

https://www.acronis.com/en-us/support/

At a minimum the chipset drivers should be installed to allow functionality with windows 10.

Oem Audio driver is always recommended.

https://downloads.dell.com/FOLDER03113490M/1/XPS-8700_Audio_Driver_MN0T2_WN32_6.0.1.7525_A00.EXE

Bios likely needs update

https://downloads.dell.com/FOLDER03163439M/1/XPS_8700_BIOS_A11.EXE

chipset driver installed is always minimum required.

otherwise devices may not enumerate properly including USB3

https://downloads.dell.com/FOLDER03130099M/1/XPS-8700_Chipset_Driver_CR5PJ_WN32_10.1.1.7_A00.EXE

HECI Chipset driver

https://downloads.dell.com/FOLDER03130113M/1/XPS-8700_Chipset_Driver_VJ54R_WN32_11.0.0.1153_A00.EXE

Again the "Acronis True Image" Boot media is not Windows 7 or 8 from Dell,  Nor is the software supported by dell or guaranteed for fitness for any particular purpose.  There is no service or support for this on the Dell forum as it is not Dell software. Neither Dell Nor Microsoft support Acronis imaging.

https://www.acronis.com/en-us/support/

MuddyR, there are lots of USB speed test software available; just Google search "usb speed test windows 10". While maybe not Dell or Microsoft supported I think is will be adequate for what you are doing, trying to why your USB 3.0 ports are operating at USB 2.0 speeds. The ratio of USB 3.0 to USB 2.0 speed should be about 10, but will be limited by the device you are using to conduct the test unless you use an USB loopback plug.

Real world test. Copy 12 Gigs of random files from downloads to WD 1 TB USB3 hard drive. Random files of all shapes and sizes. Took 4 minutes exactly, yielding a transfer rate of 52.7 MB/s which is not 10X usb 2 speed.  You cannot be faster than what you are reading from and writing to. 

When you transfer files, the likely path is the following:
Internal SATA HD -- SATA cable -- SATA controller -- CPU -- Windows' USB stack - USB3 host controller -- USB3 cable -- USB3 controller -- USB3 to SATA bridge -- SATA HD.

This is a complex system, not just a single interface (USB3), and you would need to test each piece separately to figure just who or what is responsible for the less than advertised throughput.

120 MB/sec comes to about 1 Gbps. In no way should that be considered "slow".  Sata 3 speed even with SSD's is not 600Mbps.

USB3 hard drive tests.

The title of the post suggest concern about the speed of USB 3.0 ports relative to USB 2.0 ports. Most of us know that USB speed depends upon and is limited by the USB devices connected to it. The only way to test USB speed without USB device limitations is with an USB loopback plug and associated loopback testing software. Lacking a loopback test the only other way to compare USB 3.0 vs USB 2.0 ports is a test conducted with a USB 3.0 device connected to the USB 3.0 port compared to the same test connected to the USB 2.0 port. An USB 3.0 device connected to an USB 2.0 port should transfer data slower and at least prove that the USB 3.0 port does not at USB 2.0 speeds.

USB 3 connectors are NOT the same as USB 2 connectors.

USB3 being Physically incompatible with USB 2 is also an issue.

USB 2.0 based loopback has been discontinued.  USB3 Loopback seems VERY expensive.

https://www.passmark.com/products/usbloopback.htm

https://www.passmark.com/products/usb3loopback.htm

https://www.passmark.com/download/usb3test_download.htm

There are no physical compatibility issues between USB 3.0 Type A connectors and USB 2.0 Type A connectors. See www.lifewire.com/usb-physical-compatibility-chart-2624585

USB 3.0 cables are backwards compatible with USB 2.0 ports, though they will only work at USB 2.0 speeds.

Yes, but the question is if the OP is using a USB3 cable to connect a USB3 device to a USB3 port?  

Yes!  All connections are with a High Quality USB 3.0 Cable to a USB 3.0 External Drive (either 2TB, 4TB or 6TB).  Cables are Male -A to Male Micro-B..