To evaluate the RT-AC66U's performance as a network-attached storage device, I connected a 500GB Western Digital USB drive to one of the router's USB ports. I used a stopwatch to time how long it took the unit to copy a few files from a PC to the drive over the network (a write test), and then I copied a few files from the USB drive to the networked PC over the network (a read test). The PC was hardwired to the network.

I created a large-file test by ripping a DVD (Quentin Tarantino's From Dusk to Dawn) to the PC's hard drive. Copying this 4.29GB file from the PC to the portable hard drive required 289.7 seconds (about 4 minutes, 50 seconds). This was the fastest time of the five 802.ac routers I tested, but it was slightly slower than the reference RT-N66U 802.11n router. The D-Link and routers were off the chart here, with scores of 1233 and 2211 seconds, respectively. I couldn't benchmark the at all on this measure, because the router didn't recognize my NTFS-formatted hard drive.

Surprisingly, the RT-AC66U was slower at copying (reading) the large file from the USB drive than it was at writing to the drive. On the other hand, as the chart below makes clear, the two Asus routers were faster than most of the rest of the field on this measure.

Unless you rip a lot of movies from DVD or Blu-ray discs, you'll rarely move a single large file to a hard drive attached to your router. A more common task is to move batches of small files back and forth across your network. To evaluate each router's performance in this scenario, I created a single folder containing 595MB of small files (subfolders containing music, graphics, photos, documents, spreadsheets, and so on).

On this task, the RT-AC66U delivered the fastest write performance of any of the 802.11ac routers I tested; it was bested only by the Asus 802.11n router I used as a reference point.