I've installed OpenWRT 18.06.1 on a freshly bought Linksys WRT1900ACS a few hours ago. So far I'm having issues with WiFi. Here's what happens:
On both 2.4 and 5GHz, download speeds are as expected, but upload speeds get capped at around 2 Mbps. However, if I disable WMM, download speeds go down (expected) and upload speeds go up. Over ethernet I always get the expected speeds.
Some example numbers, from speedtest.net, with my laptop (a 2015 MacBook Pro) right in front of the router:
- WMM enabled: ~120 Mbps down / ~2 Mbps up
- WMM disabled: ~20 Mbps down / ~10 Mbps up
My internet connection is capped at 120 Mbps down / 10 Mbps up, which is what I see if connected to an ethernet port on the router.
With WMM enabled my mac is connecting at 600 Mbps, and with WMM disabled it's connecting at 54 Mbps, both as expected.
Now for something weirder: using iperf I get around 450 Mbps both ways.
With my older Linksys E3200 (running the stock firmware) I have none of these issues.
EDIT (November 25, 2018): The fix for this turned out to be enabling QoS/SQM. Capping the connection to the ISP-advertised speeds was enough, no need to take a few percent off as recommended in the wiki (although, to minimize bufferbloat, I've since done that).
I'm still struggling to understand what was happening, especially as to why only wireless clients are affected, but definitely my ISP-provided router is (was) also conditioning traffic and that's critical somehow.