Samsung 950 Pro m.2 NVMe Slow Sequential Read

SPARTAN VI · November 7, 2016

Built a buddy of mine a wicked gaming rig and it's about 99% dialed in, except for the Samsung 950 Pro has sequential read speeds that are about 1/3 where they should be (700-800MB/s vs 2200MB/s). This is the 3rd time I've installed a m.2 SSD on a Skylake build this year, so I thought I was ahead of this problem by disabling CSM and setting up Win10 using GPT instead of MBR. I'm on the verge of returning the 950 Pro for a standard 2.5" SSD instead, but wanted to run this by the LTT community first.

Here are his specs:

CPU: Intel Core i7-6700K 4.0GHz Quad-Core Processor
CPU Cooler: Corsair H100i v2 70.7 CFM Liquid CPU Cooler
Motherboard: Asus MAXIMUS VIII HERO ATX LGA1151 Motherboard (BIOS v2202)
Video Card: ASUS GeForce GTX 1080 8GB ROG STRIX Graphics Card (STRIX-GTX1080-8G-GAMING)
Memory: Corsair Vengeance LED 32GB (4 x 8GB) DDR4-3200 Memory
Storage: Samsung 950 PRO 256GB M.2-2280 Solid State Drive
Storage: Western Digital Blue 3TB 3.5" 5400RPM Internal Hard Drive
Case: Phanteks Enthoo EVOLV ATX ATX Mid Tower Case
Power Supply: SeaSonic X Series 850W 80+ Gold Certified Fully-Modular ATX Power Supply
Wireless Network Adapter: Asus PCE-AC88 PCI-Express x1 802.11a/b/g/n/ac Wi-Fi Adapter

What I've tried so far (no particular order):

PCIe set to Gen 3 speeds. Note: Asus's manual doesn't describe how to toggle SATA or PCIe modes.
BIOS update to 2202.
Installed NVMe driver.
Disabled CSM. Installed Win10 w/ GPT partitioning.
Formatted and reinstalled Win10 w/ GPT partitioning.
AS SSD Benchmark and Samsung Magician both report the sequential read is about 700-800MB/s and random read IOPS are about 80% as advertised. Oddly the sequential and random write speedsare very close to advertised speed.