It appears to only impact download speeds on iOS devices. Upload is unimpacted. I can confirm 12.1.4 was OK with firmware 10.2.0.0.189. 24 hours ago, I was running speedtest with 450Mbs downloads and now downloads are 30Mbs on the same device running the same iOS in the same location on the same AP.
The firmware that works is 10.2.0.0 build 189. Everything after that shows the problem of slow downlink.
ZD on firmware version 10.2 build 189 with 4 AP's (R710), speed in Iphone download speed:500mbps.
After upgrade the speed is download speed:95mbps, the issue is only with download speed and on Iphones.
The wlan is wpa2(aes).
no rate limit configured.
upgrade to 10.3 build 398.
Create a new open wlan and the devices are getting good speed 500mbps, however when we change the Encryption type to wpa2(aes) the speed again drops.
Change the Encryption type to wpa2(TKIP+AES), still the same speed.