How can sysbench fileio throughput be only 6.92MiB/s on fast NVMe disk?

[sunknudsen]

I followed guide to benchmark WD_Black 1TB SN750 SE NVMe disk (read: 3600 MB/s, write: 2830 MB/s) on HP EliteDesk 800 G2 (i5-6500T).

$ sysbench fileio --file-total-size=8G prepare
…
8589934592 bytes written in 7.91 seconds (1035.26 MiB/sec).

$ sysbench fileio --file-total-size=8G --file-test-mode=rndrw run
…
Throughput:
    read, MiB/s:                  6.92
    written, MiB/s:               4.61

$ sysbench fileio cleanup

$ dd bs=512k count=2048 if=/dev/zero of=/root/data
2048+0 records in
2048+0 records out
1073741824 bytes (1.1 GB, 1.0 GiB) copied, 0.817983 s, 1.3 GB/s

$ sysctl -w vm.drop_caches=3

$ dd bs=512k count=2048 if=/root/data of=/dev/zero
2048+0 records in
2048+0 records out
1073741824 bytes (1.1 GB, 1.0 GiB) copied, 0.497434 s, 2.2 GB/s

How can throughput be so slow given fast NVMe disk?

Did I hit bug or edge case?