It may very well turn out that the D-Link DIR-820L has a crappy 802.11ac AP implementation. Most 802.11ac APs are top-of-the-line, flagship devices for around $180, that hit the 1300Mbps data rate, which requires:
- 3 spatial streams
- VHT80 (80MHz-wide channels)
- MCS 9 (256-QAM)
- short guard intervals (SGI)
Slightly less expensive ones only hit the 867Mbps data rate, which requires all of the above except just 2 spatial streams instead of 3.
This device only does a 650Mbps data rate, which probably means that not only does it not do 3 spatial stream, it also doesn't do MCS 8 or 9 (256-QAM).
Note that I'm only talking about the 5GHz 802.11ac performance by itself, I'm not falling for the marking BS where they add their 2.4GHz 802.11n performance to their 5GHz 802.11ac performance.
Anyway, the fact that they can't do 3 spatial streams and can't do 256-QAM is a sign that they made some deep compromises to hit that $75 price. I also note that it only has 100Mbps, not Gigabit, Ethernet LAN ports. Further evidence of corner-cutting that hurts performance.
To really see what that AP is capable of, you need to do a performance test that eliminates the overhead of disk I/O and remote filesystem protocols like SMB and AFP.
I usually recommend that one endpoint of your performance test be on a wired Ethernet LAN port of the AP, but since this AP's wired Ethernet isn't Gigabit, that'll be the bottleneck. The best you could possibly do is 94 Mbps = 11 MiBytes/sec.
So to see something better than 11MiB/sec, you have to do a wireless-to-wireless test, but that will cut your Wi-Fi performance in half.
So anyway, associate your 802.11ac iMac and 802.11ac retina MacBook Pro to the 5GHz network, and run IPerf on each. Make the iMac the IPerf server, and make the retina MacBook Pro the IPerf client. On both ends, add the
-w 2M option to the IPerf command-line so that you have a nice big TCP window to work with.
Make sure all three devices are at least 2m apart from each other so they aren't saturating each others' receivers.
If all goes well, and they're all able to sustain the 650Mbps signaling rate that your AP tops out at, I'd expect you to see a max throughput of 260Mbps, which is 31MiB/sec.
The calculation goes like this:
- Take the signaling rate (PHY rate) the devices are able to sustain.
- Divide it in half since both devices are sharing the wireless medium and the AP is having to do Intra-BSS Relay, which means every packets is sent across the channel twice; once from the source device to the AP, and then again from the AP to the destination device.
- Take 80% of that (i.e. multiply it by 0.8) to account for Wi-Fi overhead, assuming frame aggregation (A-MPDU).
So (650/2)*0.8 = 260Mbps. 260/8.4 = 31MiB/sec (8.4 is to convert from 10^6 bits to 2^20 Bytes).