Why is Target Disk Mode so slow?
For the touch-bar 13" MBP the right side ports have reduced bandwidth, this could explain a portion of the difference in speed between the two modes. However, you will likely never see the full speed while running in TDM, that is because TDM runs as a UEFI application within the UEFI rom, this limits the resources that can be used in targeted disk mode. Thunderbolt bridge should get you full speed, if that is an option for your use case. 


  Other boot-related features, such as Target Disk Mode, the Startup
  Manager, and the firmware password user interface, are all implemented
  as UEFI applications built into the main UEFI firmware.