Done all you suggested still no changes. See my ansers below:
Ensure the default (dlink) SSID name is changed. Ensure that different SSID names are used between each WiFi 2.4/5Ghz radios. Can be anything and not something that's already in use by any neighbouring WiFi routers. Under Setup/Wireless/Manual. - already done that when configuring
What wireless modes are you using? - I tried all mdes, no change
2.4Ghz Routers only: Try single mode G or N or mixed G and N? - i tried that, no change
Channel Width set for Auto 20/40Mhz or try 20Mhz only. - done that, no change
Try setting a manual Channel to a open or unused channel. 1, 6 or 11. 11 for single mode N if the channel is clear. 13 for EU regions. - tried with channel 1, 6 and 11 as manual channels, still no improvement
What security mode are you using? Preferred security is WPA-Personal. WPA2/AES Only. Some WiFi adapters don't support AES, so you might want to try TPIK only or Auto TPIK and AES. I am using WPA personal. I played with all the security settings to see if there is a change but np result
What wireless devices do you have connected? - my laptop and a few mobile phones ( android and iPhone )
Ensure any devices with WiFi adapter drivers are updated. all devices have updated drivers, including the laptop I am checking the speed on.
Any 2.4Ghz or 5Ghz cordless house phones or WiFi APs near by that maybe causing interferences? - no cordless phones or WiFi AP
Any other WiFi routers in the area that maybe causing interference? Link> Use InSSIDer to find out. How many? Use v3, its free. - yes, many routers in the area as I live in a skyscraper with many apartments. InSSIDER can pick up another 8 wireless devices and it shows Co-channel 1 network and overlapping 1 network. this is with channel 11 manually chosen on other channels it's more crowded
If you have any of these options, Try turning OFF or ON Short GI, WLAN Partition, Extra Wireless Protection and HT 20/40 Co-existence if you have it. Also testing with HT20/40Mhz Co-existence enabled will impact results as well. I prefer to use this option OFF. Recommended settings are default. Under Advanced/Advanced Wireless. - dont' have these options.
I am running out of options here, still can't understand the drop in speed.

Anu other ideas?