My understanding AC3000 = 450 + 1300 + 1300. and your individual client device can only connect to one of the bands and have 3 antennas to take full advantage of the bandwidth. Most client devices only have 2.
And as suggested, wifi depends on a lot of factors (line of sight, wall materials, environment noise, distance, location, etc. ). The wifi signal strength will fluctuate 24/7.
Your Nighthawk® X6S Tri-Band WiFi Router spec. or EX8000 extender, should be similar or the same.
https://www.netgear.com/media/R7900P_tcm148-54005.pdf P.12
Technical Specifications
• AC3000† WiFi-
Band 1: 450 Mbps @2.4GHz-
Band 2: 1300Mbps @5GHz-
Band 3: 1300Mbps @5GHz
• Simultaneous Tri-Band WiFi-
Tx/Rx 3x3 (2.4GHz) +
3x3 (5GHz) +
3x3 (5GHz)-
IEEE® 802.11 b/g/n 2.4GHz
IEEE® 802.11 a/n/ac 5GHz
Even if you know the link speed, it's not the real speed.
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@bill001g for details.