Dell XPS 15 9530 brand new (Dec 13) - SSD reports latency around 30ms with minimal activity - slow performance all round

How are you measuring latency?

I have a Samsung 940 EVO in the SATA bay -- using Samsung Magician I measure 553 MBs read and 525 MBs write.

My understanding is the mSATA yours shipped with is based on Samsung Pro -- you should see comparable read speeds but slower write speeds.

Samsung is about to come out with the EVO in mSATA form factor, so you could get a bigger faster drive.

Mine has a Micron C400 in the mSATA slot -- it reads 462 MBs read and only 56 MBs write. But its a much slower controller.

You might want to check that the BIOS is in AHCI or RAID mode -- if its in ATA mode you will get much slower performance from your SSD.