WiFi by it's nature is generally going to be slower then most any wired network. Not only do you share the bandwidth with everything else attached to that WiFi access point, but you can also get slowed down by your relative position in relation to the antennas and what is between you and the antennas (Walls, steel, sheet-rock, whole buildings, etc.)
First it would be helpful to know what the make and model of the WiFi hardware is that you are connecting to, where you are in relation to the device and what is between you and the device.
Also you may want to try a different access point. Possibly at work, a friend's house, a local coffee shop or other public location with WiFi.
With WiFi there are a lot of variables and before you go looking into problems with your Mac (possible but less likely than other things) you need to see if the problem exists connecting to other access points or even if just moving closer to your current one solves it.
Additionally I would be tempted to look in both /Library/Preferences and ~/Library/Preferences for any network/WiFi related preference file. Delete them and reboot. Then set up your networks again: you might have a damaged prefs file.