You aren't the only one who has seen this problem.
I have two Samsung t7 "shield" drives (each 1tb) and they exhibit the problem.
Typically, when I first connect the drive, and try a speedtest on it (before doing anything else), it will offer up fast read speeds, but write speeds are TERRIBLE (as seen in the OP's post above). Both my drives are formatted HFS+.
Here's an "AJA System Test Lite" reading I just recorded while composing this reply:
There's SOMETHING -- whether it be in the drive's circuitry, or elsewhere -- that is failing to establish proper communication speeds with the Mac when first connected.
I happen to have an old copy of Drive Genius, and it offers "repair" and "rebuild" options for drives (the latter being a modest "directory rebuild", not sure?).
I opened DG3, and ran the rebuild option on the drive.
Here are the results:
===============
Repairing volume...
Checking Journaled HFS Plus volume.
The volume name is t7 Movie & TV Archive
Checking extents overflow file.
Checking catalog file.
Checking multi-linked files.
Checking catalog hierarchy.
Checking extended attributes file.
Checking volume bitmap.
Checking volume information.
The volume t7 Movie & TV Archive appears to be OK.
Rebuilding volume...
Checking Journaled HFS Plus volume.
The volume name is t7 Movie & TV Archive
Checking extents overflow file.
Checking catalog file.
Rebuilding catalog B-tree.
Rechecking volume.
Checking Journaled HFS Plus volume.
The volume name is t7 Movie & TV Archive
Checking extents overflow file.
Checking catalog file.
Checking multi-linked files.
Checking catalog hierarchy.
Checking extended attributes file.
Checking volume bitmap.
Checking volume information.
Invalid volume file count
(It should be 1338 instead of 1316)
Invalid volume directory count
(It should be 271 instead of 270)
Invalid volume free block count
(It should be 106642522 instead of 106838533)
Volume header needs minor repair
Repairing volume.
Rechecking volume.
Checking Journaled HFS Plus volume.
The volume name is t7 Movie & TV Archive
Checking extents overflow file.
Checking catalog file.
Checking multi-linked files.
Checking catalog hierarchy.
Checking extended attributes file.
Checking volume bitmap.
Checking volume information.
The volume t7 Movie & TV Archive was repaired successfully.
Verifying volume...
Checking Journaled HFS Plus volume.
The volume name is t7 Movie & TV Archive
Checking extents overflow file.
Checking catalog file.
Checking multi-linked files.
Checking catalog hierarchy.
Checking extended attributes file.
Checking volume bitmap.
Checking volume information.
The volume t7 Movie & TV Archive appears to be OK.
===============
Note the following that was found:
"Invalid volume file count
(It should be 1338 instead of 1316)
Invalid volume directory count
(It should be 271 instead of 270)
Invalid volume free block count
(It should be 106642522 instead of 106838533)
Volume header needs minor repair
Repairing volume."
But... over the course of months... the problem will "be back" each time I connect the drive. The "fix" doesn't "stick".
HAVING SAID ALL THAT...
I've found that if you "plow through" -- that is, cancel and then restart the test several times -- the write speeds will suddenly "jump upwards" to where they SHOULD be.
Here's what happened if I stopped, and then restarted the AJA test 3 or 4 times:
Not sure if this matters with APFS formatted drives or not.
My other t7 has a single APFS partition on it, but it's not near at the moment.
My advice to the OP:
Try AJA System Test Lite (free)
or
Try Blackmagic Disk Speed Test.
If the initial write result is bad, stop and restart the test.
Repeat this about 4-5 times.
See if the write speeds suddenly "break through" and jump up to where they should be...
