I've reached out to Netgear to see if they might have an explanation. If a router with high NAS performance is important to you, the Linksys WRT1900AC remains the best solution. And if high NAS performance is super important, you'd still be better off with a dedicated solution (I particularly like WD's My Cloud Mirror).
Should you buy one?
If you absolutely must have a Wave 2 router, Asus's RT-AC87U beats the stuffing out of Netgear's Nighthawk X4, especially when servicing a client attached to a second identical router configured as a wireless bridge. Asus's router doesn't have the Nighthawk's fancy adaptive QoS, but it's significantly faster in every single benchmark I ran.
But a more important question might be "Should you buy any Wave 2 router today?" You see, multi-user MIMO support is one of the most important features that Wave 2 802.11ac routers are supposed to deliver, and while Quantenna's chipset is capable of MU-MIMO, it's not enabled in the firmware of either router right now. Netgear's Harpalani told me that "... is expected before the end of the year." When you're paying top dollar for a router in September, the end of the year is a long time to wait.
And while both routers support four spatial data streams, you'll realize that benefit only when you have a 4x4 client device. When tested with 3x3 802.11n and 2x2 802.11ac clients, several of the first-generation 802.11ac routers delivered higher performances than both of these Wave 2 models.
I'll happily re-test the Nighthawk X4's performance if Netgear comes up with new firmware that remedies the performance shortcomings I've reported here. In the meantime, the Asus RT-AC87U remains at the top of my leaderboard, with Netgear's Nighthawk X6 and the Linksys WRT1900AC running neck and neck for second place. If all those choices are outside your budget, the Asus RT-AC68U and the Netgear Nighthawk (R7000) deliver a lot of band for the buck. D-Link's DIR-880L is a good router, too; I just don't like the way it handles network-attached storage.
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