I had the same issue with a $300 netgear prosafe AP. When certain 802.11ac clients would connect (mostly cell phones), it would randomly get periods of of complete grid lock in the wlan. All clients would have good signal strength but they had no data throughput. Disconnecting and reconnecting the offending client would restore the wlan throughput.
The best I could see from packet sniffing was that these certain mobile clients would go into some kind of sleep mode, and when/if packets were transmitted to them, they would not respond with the proper ack (sometimes), the AP would then hang the whole wlan waiting for the ack (I guess). I tried several different netgear prosafe access points (all of them 802.11n). The only thing that worked was getting rid of my netgear hardware and replacing them with a Mikrotik wAP ac.
The best I could see from packet sniffing was that these certain mobile clients would go into some kind of sleep mode, and when/if packets were transmitted to them, they would not respond with the proper ack (sometimes), the AP would then hang the whole wlan waiting for the ack (I guess). I tried several different netgear prosafe access points (all of them 802.11n). The only thing that worked was getting rid of my netgear hardware and replacing them with a Mikrotik wAP ac.