Mac book Pro is very slow on wifi connection
Excellent work with the details. The transmit rate and MCS index indicate that your mac has a high-speed connection to your router and that it's not an interference issue.

My next step would probably be timing me file transfer using Wi-Fi from the Mac to the PC or vice versa. You can enable filesharing on the Mac quite easily. download some YouTube movie from the Internet so that you've got something that will take 15 to 30 seconds to transfer so you can time it with the stopwatch. 

It's hard to guess whether flash that runs the speed test.net simply running slow or you've actually got a legitimate speed issue with the Mac. 

You'll also want to watch the activity monitor and the console application to see if you have io errors and if the network throughput matches the disk io which should all matching your benchmarking test.