Support asked what the free space was, I said it was 60% full. They then said my question was being sent to another department.
As a test, I tried removing a large amount of data from the drive and putting it back on. Here were the tests:
1) Starting test:
2) Removed over 400GB of data, expected speeds returned:
3) Added some data back (call it folder 1), speeds are gone:
4) Added more data back (folder 2):
5) Tried removing data put back in step 3 (folder 1):
6) Left it for a few days, a few reboots, etc...:
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Not sure if it's the expected results of the speeds to disappear at that capacity, or if the drive is defective.