increase digicom wireless router speed WLAN

how to increase WLAN Speed for Digicom Routers

I have a digicom router and its speed is very low when I surf internet with WLAN and its speed is fair when I surf with a network cable. So I do not know what the problem is but the case is that when I set the wep key for the router, its speed decrease and when I let it open network then its speed is fair.
I do not know why.

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