What is the current signal strength that the client receives? SNR is the key to speedy connection rather than just signal strength. If you are using 2.4GHz and signal strength is acceptable level you may need to check for interference.
If you are using 5GHz network and are in a reigon that allows for TPC within the DFS channels then you could look at changing the channel to one which allows a higher EIRP under regulation and set the power to full. However, you need to check the devices supported by this AP support those channels and also check the max TX power of the client adaptor too.