I noticed the same issue today , write speed @ 1 Gbyte/s. Was running fine before.
After deleting DCS my free space went to 700GB, speed back to normal.
PCIE 4.0 doesn't seem that useful anymore now :(
3900x, Gigabyte X570 Master BIOS F7b, 2x16GB G.skill NEO 3600 16-19-19-19, Win10 1903, latest chipset drivers
MP600 1TB SN 19288290001285537BA EGFM 11.2
Used as OS Drive, first NVME (direct to CPU) slot, standard Corsair cooler, temps < 50C
I'm afraid it might be a 'hidden' feature but am hoping it's a FW Bug really.