Asus rt-n66u iphone 4s and 5 speed troubles

[frixifrax]

Hi. I have Asus rt-n66u and 90 mbit internet at home. Problem is when i try to get wifi via phone it shows speed 1-3 mbit only... why? my ipad mini retina shows 60mbit on same wireless network. Maybe anyone can help me. I've also tested it with iphone 4s - it shows same reulst 1-3 mbit only. Why speed is sooo low? Any ideas how to fix it?

Asus Firmware Version:3.0.0.4.374_5517 updated today(5.05)

My home-pc via network cable connected to Asus rt-n66u:

iPhone 5... 2.4GHz network:

iPhone 5 via 5GHz show results: ping 158, DL:3.98, UP: 22.48

iPhone 4s... 2.4GHz network:

iPhone 4 doesnt see 5GHz network

iPad mini retina 5Ghz:

iPad mini retina 2.4Ghz show results: ping 7, DL:3.57, UP: 64.55