OK, this is unbelievable, but resetting the mainboard worked. Thank you for the suggestion. I had already bought a new disk and ram…guess I got to return them now.
$ sudo hdparm -tT /dev/nvme0n1
/dev/nvme0n1:
Timing cached reads: 35536 MB in 2.00 seconds = 17808.56 MB/sec
Timing buffered disk reads: 6778 MB in 3.00 seconds = 2259.21 MB/sec
$ sudo hdparm --direct -tT /dev/nvme0n1
/dev/nvme0n1:
Timing O_DIRECT cached reads: 4650 MB in 2.00 seconds = 2325.50 MB/sec
Timing O_DIRECT disk reads: 5978 MB in 3.00 seconds = 1992.43 MB/sec