macoswifi
I currently have a MacBook Pro (Lion) and my iMac (Leopard) sitting side by side. The MacBook Pro connects to a wifi network and speeds are as they should be. The iMac has serious problems: it doesn't find the same networks, it gets connection timeouts when trying to connect to the wifi network, when it finally connects the speed is so slow that it's unusable. When I ping yahoo.com I get results of between 100 – 1500ms and sites take minutes to load.
How can I start debugging this problem?
The wifi icon (click here to see how you could have found about it on your own) is the top bar, right between the carrier and the clock, and it replaces 3G or E. It looks like this: 
Like Alvar said, best way to be sure is getting in Airplane mode and turning wifi on.
As for youtube not working while others do, it's probably blocked. This could be on the wifi or on the iPhone, if it's a company phone with a provisioning profiles blocking it or if it's jailbroken and got a firewall on it. Just check if youtube works on any other device on the same wifi and you'll know for sure where's the issue.
If youtube is just running slow, but it is running, it could also be your other heavy sites are cached. Without more info I can't think of more options for that issue.
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.
Best Answer
Here is a general plan to start debugging this problem. Post your results and we'll help you figure this out. I'm running an iMac with Snow Leopard and these steps have helped me.
Write down key data for each machine for each wifi connection. I've included my data below.
Go to the iMac and hold down the option key while clicking the Airport icon in the menu bar. Write down the PHY, BSSID, Channel, RSSI and Transmit Rate.
Move over to the MBP and gather the same information.
Phy Mode this tells you if you joined the network as a B, G or N connection. I'm on an Airport Extreme N network using the 2.4 Ghz frequency.
BSSID - This is the access point you are logged into. They are likely the same on each machine. If they are different, your machines are logged into different access points.
Channel - This is the channel each machine is using. In my case, my neighbor and I were both on channel 1. I changed over to 11 and my speed improved since I was no longer fighting his interference.
RSSI - Received signal strength. Mine is -43 while my neighbor's is -88.
Transmit Rate - The higher the better. Mine is 130. I can't view the neighbor's rate but it's like less due to his lower RSSI.
If you see other nearby networks, you can view some information about their signal by moving your cursor over the other network and a small dialog box will pop up with their PHY, BSSID, Channel, and RSSI. This is how I found my neighbor was using the same channel as me.
You can also view this data in System Profiler under Network>Airport.