Since router performance can be greatly influenced by atmospheric and environmental conditions, I always run fresh benchmarks for each router comparison I conduct, ideally testing all routers I'm going to write about on the same day (after updating the router's firmware and the OS on the client and server).
That's what I did in this case, using AccessAgility's WiFiPerf to measure TCP throughput. As you can see in the benchmark charts, the Asus RT-AC66U delivered better TCP throughput at most locations inside my home. I suspect the Asus's higher performance stems from its use of external dipole antennas that can be positioned to achieve the best reception.
After testing the updated 802.11ac Time Capsule, I went back and compared its performance to the numbers I recorded in July. The router's performance in the close-range test (with the MacBook Air and the Time Capsule in the same room, about nine feet apart) was roughly the same: 458.1 mbps now compared to 451.9 mbps then. And its TCP throughput was slightly faster when the client in the kitchen, about 20 feet from the router: 405.95 mbps now compared to 387.2 mbps then.
Curiously, I measured much lower TCP throughput this time when the client was in my home theater (35 feet from the router) and in my home office (65 feet from the router): 73.7 mbps now, compared to 100 mbps then for the home theater; and 53 mbps now, compared to 167.8 mbps then, for the home office. (If you're wondering why the numbers for the home office are higher than they are for the home theater, despite the longer distance, it's because the home theater is double-walled and double-insulated to enhance its acoustic properties.)
I repeated these tests several times to ensure I wasn't seeing an anomaly, but the results were consistent. Fortunately, the 802.11ac Time Capsule delivered excellent performance just about everywhere else in my house (see the illustration of my home's floor plan for details). I don't know if the new operating system and router firmware are responsible for this reduction in throughput.
I suggested in my original review that Apple would solve the wireless file-transfer issues with a software update, and that expectation was baked into the 802.11ac Time Capsule's score, so nothing changes there. I'm glad they didn't make everyone wait for Mavericks to do it.
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