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There are a lot of factors as to why a wireless connection can be slowed down..
Is the router underneath anything? Where is it positioned?
How far away from the router are you when you receive 1.5mbps?
The symptom sounds like wireless channel interference. Did you try changing the wireless channel to another "far away" from the previous one?
> If wired it works fine like 25 mbps but when I go wireless it drops to 1.5 mbps.
I had the exact same issue with my Linksys router. The issue was fixed after changing the wireless channel and bandwidth.