Why is Target Disk Mode so slow? Two macs:


maxed out 2017 MacBook Pro (MBP)
base model iMac Pro (iMP)


Using the MBP standalone, the internal drive gives me read &amp; write speeds of about 2600 &amp; 2000 MB/s respectively.

Using the iMP standalone, the internal drive gives me read &amp; write speeds of about 2500 &amp; 2900 MB/s respectively.

All pretty darn good, obviously.

Now, in separate tests, I connect each to the other in Target Disk Mode over Thunderbolt 3 (using a short 1 foot cable so as not to lose the maximum 40Gb/s speed).

Booting the iMP from its internal drive and then connecting the MBP in Target Disk Mode, testing the MBP (Target Disk) I get read &amp; write speeds of about 450 &amp; 700 MB/s respectively.

Then the reverse: Booting the MBP from its internal drive and then connecting the iMP in Target Disk Mode, testing the iMP (Target Disk) I get read &amp; write speeds of about 200 &amp; 250 MB/s respectively!!

What on earth is going on?  Thunderbolt 3 in real world tests with any other fast enough drive is capable of at least about 2800 MB/s.  Does anyone know why Target Disk Mode is so slow?  

I understand there's the obvious speculative answer of "Well something else relating specifically to Target Disk Mode is obviously bottle-necking it."  Ok... obviously.  But I'm wondering if anyone knows anything specific on the technical level of what's going on, and if there's anything that can be done to improve it.

Given the iMP drive over Target Disk Mode being so slow, even significantly slower than the MBP when really it should be the reverse, I'm guessing there's something the T2 chip is doing that's bottlenecking it, but again, just wondering if anyone knows anything official or definitive, rather than just speculation.

Thanks!
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.