RTN-66U "dark knight" on 5ghz 2 devices make it slow?

Hey guys new to this form just wanna tell everyone hi!

just bought a iPhone 5 last night
and when it connects on my 5ghz network on my Asus rtn-66u
it slows down my iMac and my iPhone
i have a 50 mbps connection from Comcast

and when i connect 2 devices its slowing it down so bad that i can't even go to google to search anything

unless i put my iPhone on 2.4 network

then every thing is fine again

iPhones
ip address is 192.168.1.5
Subnet mask is 255.255.255.0
Router 192.168.1.1
dns is 192.168.1.1

iMac
ipv4 192.168.1.2
subnet mask 255.255.255.0
Router 192.168.1.1
dns 192.168.1.1

Router settings
Wireless Mode auto
Control Channel auto
Channel bandwidth 20/40
Extension Channel: auto
Authentication Method WPA2 personal
WPA Encryption aes
network key rotation interval 3600


Thanks guys Appreciate any help and comments back
Have a good day guys!