Poor (~350 MB/s) sequential write performance on XPS 13 9370 / SK hynix PC401 NVMe 512GB / Ubuntu 18.04 / ext4

Hi, my XPS 9370 has an SK hynix PC401 NVMe 512 GB SSD. I'm using the preinstalled Ubuntu and in short, getting a fraction of sequential read/write speed that I expected.

$ dd if=/dev/zero of=/tmp/out.bin bs=1M count=8192 # write perf.
8192+0 records in
8192+0 records out
8589934592 bytes (8.6 GB, 8.0 GiB) copied, 24.4776 s, 351 MB/s
$ dd if=./tmp/large.bin of=/dev/null bs=1M # read perf.
6700+1 records in
6700+1 records out
7026150816 bytes (7.0 GB, 6.5 GiB) copied, 22.2478 s, 316 MB/s

So, read and write performance in the 300+ MB/s range.

There are SSD benchmarks suggesting seq. write speeds of 1.2 GB/s, 1 GB/s and 0.7 GB/s: all of these are much faster than the 350 MB/s that I get (at best).

There is even a thread here on the forum describing 800+ MB/s speeds as normal/expected.

The drive is set up to work in AHCI mode.

Is this disappointing performance a matter of malfunction, misconfiguration or misunderstanding?