I would try replacing the AirPort card cable. Follow this IFIXIT guide: MacBook Pro 15" Unibody Mid 2012 AirPort/Bluetooth Cable Replacement and this is the part you'll need: MacBook Pro 15" Unibody (Early 2011 through Mid 2012) Airport/Bluetooth Cable - Apple P/N 922-9750
@ellyyogini - You made an interesting statement here "But as soon as the MacBook Pro got online, everything slowed from a previous 51 Mbps to about 6 Mbps download" It sounds like your MacBook Pro system is saturating the WiFi channel, which had me thinking. If you have something going on like a software update in process.
Install this app on your MacBook Pro: Network Speed Status it will allow you to see what your data flows are coming and leaving your system. If you are not doing anything on your system the data rate should be quite low. If you are seeing something going on double check your App Store Updates. If nothing is downloading then you may have a DoD bot sitting on your system which is why your network bandwidth is being taken up. You'll want to review what processes are running on your system using Activity Monitor. Click on the Network tap to see the top sender and then compare what your iMac has running assuming they are running the same OS version and apps by default.
I'm also wondering if you have a bad config on one of your systems or Router. Install this on your iMac from the Apps store: WiFi Explorer so you can see whats happing within the WiFi signals. You may want to do some alterations as I describe below.
Using 'WiFi Explorer' will allow you to see each of the bands your Router is offering out as well as your neighbors as well. You may want to alter the frequencies you are using so they are not on top of what your neighbors are using.
You might want to alter the Routers AP naming for the different bands. As an example if your router is called ACME then both the 2.4 & 5 GHz bands see it as the same AP name (ACME). Instead rename the radios by appending the band number to each so you would have ACME 2.4 & ACME 5. This will make isolation of issues easier and allow you to pick the best connection for the given device and its location.
You also want to make sure all of your systems are using a WPA/WPA 2 connection to your router and block the public open connection. That way your WiFi access is private and no one else is using your Internet access & bandwidth.
In any case I would still replace the cable so that is rules out. FYI: you can also download the apps to your other Mac system both can be useful both at home and on the road.