Your issue might be more basic than what i read in previous posts. I had the exact same issue. My Dell laptop was at 10mbps+ when my mac was at 2mbps. The issue is that your Mac will switch network. If it picks a signal from another SWP (unsecured, or one that you have connected to before with a password, like your neighbor's), it might very well connect to it.
Here is the fix:
Click on the internet label on the top right, it will show you what you are connected to, is it your network, or something else ..OR Go to the Apple symbol up in the left corner, select "system preference"., select "network", then select "advanced" in the bottom right corner. A list of preferred network in displayed. I recommend eliminating all networks except yours. To delete other networks, click on it and select the "-" button. Once you have done that, select "ok", then "apply", close all windows.
I just did that a few minutes ago and my internet speed went to the right one.
Good luck, please let me know if this worked.
Note: I ran the speedtest.net with 2 other cellphones and 1 other computer connected and it was fine