Slow write TB3 EVO+ speed on iMac 2019 i9 vs 2018 MBP i7

It’s what I would expect! Your source drive is likely the limiting factor.

You are hitting either the limits of the SATA interface 6.0 Gb/s of the 2 TB SATA Samsung SSD. You do have a faster SSD the PCIe/NVMe blade SSD, but its only 128 GB in size so that limits you on how much is holds of the test file/s.

So why is your 2018 MacBook Pro so much faster! Here Apple has done some magic which pushes the I/O to the limits on what the flash chips can run at. Unlike a typical M.2 PCIe blade SSD which uses a single channel to access each flash chip, Apple accesses each chip independently with its own I/O channel via its T2 chip controller.

I set up lot of systems for video editing for both pro’s and students.

The ideal setup is to have a sizable performance drive internally as your boot drive. Here on your iMac that would be the PCIe/NVMe drive. For small production a 500GB is fine for large production a 1TB is strongly recommended. Here we set it up as the drive holding your OS & apps as well as set for the scratch space. Basically we want to leave a good portion of the drive empty! As your OS will leverage it for virtual RAM and caching.

The second drive (SATA) is your storage of your common elements for your production and your media.

If you can’t go with this setup your alternative is to use a high speed TB drive as your boot drive, basically setting it up like I described above. This won’t be as good as the internal solution but it will also work.