In my experiance I found -- in many cases wifi dose not work or a slow connection. Possible cause may be wlan driver not partly or compatible with the device or support for 802.11b but not support 802.11g or 802.11n. This may cause for slow connection.

You may try the following step, it worked for me.

Go toterminal  alt+ctrl+T
then type:
sudo apt-get update && sudo apt-get upgrade

feel free to ask if any confusion.