Why is my wifi speed so slow but when connected to a cable it's fast?

I've been having trouble with my wifi speed. I'm on the nbn in Australia and have 93/30 speeds (paying for 100/40-so I'm happy enough) when the computer is connected directly to the nbn box. But when I've done speed tests over wifi the speeds are 21/0.01.
It's sooooo slow I can't do anything
Asked Apr 18, 2016

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