My router was set to auto-upgrade firmware and a few days ago I became unable to whatch YouTube video in 1024p and 720p on the mobile, while having no issues on PC with wired connection. That was quite strange, since i have 70Mbps internet connection and my phone shows 150Mbps WiFi connection with maximum signal strength and no other 5Ghz networks nearby.
I've checked my speed on mobile with speedtest and the result was 9ms ping with unstable 1-5Mbps download, and more stable 5Mbps upload. At the same time on PC I had 4ms ping with stable 68-69Mbps download and upload.
I've logged in to the router admin interface and noticed it was upgraded to the firmware with cloud support (I hadn't find FW version but it seems to be 2.1.41.162351).
After reverting to previous firmware 2.0.37 (thanks they had this button!) - i have stable 20Mbps download, 30 upload.
So, guys at LinkSys, please check your new firmware, why it is so slow for WiFi?!