MacBook Pro wireless speed slow after OS update
I would first try resetting your PRAM, then create a new network location.
From Apple:

Resetting PRAM and NVRAM

Shut down the computer.
Locate the following keys on the keyboard: Command, Option, P, and R. You will need to hold these keys down simultaneously in step 4.
Turn on the computer.
Press and hold the Command-Option-P-R keys. You must press this key combination before the gray screen appears.
Hold the keys down until the computer restarts and you hear the startup sound for the second time.
Release the keys.


Once you've done so, delete your network locations in System Preferences and create a new, fresh location.