ios 9.1 wifi speed drops on 5ghz

Hi,

I have a new iPhone 6s Plus and a iPhone 6, both running ios 9.1. Recently i bought a new Asus RT-87U router with a 1gbps internet connection. With the iPhone 6s Plus on 5ghz (AC wifi with WPA or WPA2), i am able to get up to 200-400mbps upload and download, running on 5ghz wifi (speedtest results). However, this speed is just for initial connection, meaning when i get connected to the router the first time after i rebooted the router and restarted the iphone, this speed can be achieved. However, when my iphone goes to sleep, and then reconnect to the router the speed will be lowered down to 50-60mbps download and 200-400mbps upload. I tried all the solutions recommended online, eg. reset network settings, forget wifi or restart iphone. This situation is exactly the same for iPhone 6. I've checked and upgraded the router firmware (tried a few other version of the firmware as well) as well, i doubt there's any issue there.

To me, it sounds like there are some issues with the 5ghz AC wifi connection with iPhone. Why the download speed gets lowered down from 200-400 to 60mbps while the upload speed remains the same at 200-400mbps after the iphone goes to sleep and then reconnect to the router ? Please advise how to solve this issue.

Thanks.