MacBook Pro Retina 2012 shows Wi-Fi speed only 11mbps

Internet speed test fast, but measured speed in system slow suggests you have added some software that conflicts or interferes with your Networking. Candidates include ANY third party apps that claim to speed up clean up, optimize or automatically do any mundane maintenance tasks, but especially Virus scanners.

Additional candidates are non-native file sync apps such as Dropbox, or one Drive, or a HUGE collection of sync-ed photos to any source.

Next is VPN or similar software that claims to make your computer invisible to others (tip: not needed, it already is invisible).

Litmus Test: try again in Safe mode. If faster, it's something you added.

Network Utility is very accurate for Ethernet speed, but it may show instantaneous Wi-Fi speed, so you should check it again. Mine shows the exact same speed as Option-WiFi Transmit rate, and has been stable for several minutes and does not sag and surge.

Speed test does a test that shows raw speed of simple data transfers.

Option Wi-Fi shows what transmit speed has been attained at this moment. Its transmit speeds tend to be very reliable and the other information is that pane can help you diagnose problems.

You probably don't need to do a Safe Mode test unless you are trying to debug a massive shortfall in speeds. Safe Mode is invoked by holding Shift at startup. It loads a small portion of MacOS, then does a five-minute disk test, then proceeds. It deliberately loads only a minimal set of Apple-only extensions, NOT including graphics acceleration extensions, and assumes defaults for everything possible. You screen resolution may revert to a lower default setting, and screen updates may be wonky and slow, but ultimately correct.

Safe mode has two additional "tricks":

• no extensions loaded means you can check for side-effects caused by third-party extensions.

• takes defaults for everything possible means you can escape from deadly wrong settings, such as screen resolution out-of-bounds, and settings you make in Safe Mode will "stick" in regular mode.

according to your option-Wi-Fi data:

You are using 802.11n modulation on channel 132 in the 5GHz band. Data channel width is 40. You have -68 signal and -88 noise for a signal to noise of 20, fair signal-to-noise.

Lower channels are somewhat stronger and travel a bit farther, so your choice of Router using channel 132 may be a slight disadvantage. However, you appear to be using three antennas and 64 patterns per signaling interval to obtain 365 M bits/sec, so there is little interference on that channel. (Your neighbors may be hogging the lower channels.)

A reliable 3 M bits/sec both ways at once is all that is needed for video conferencing.

You found it before somehow?

/System/Library/CoreServices/Applications/Network Utility/app

(Look around in that folder while you are there)

You may want to place an Alias to it in your Utilities folder.

it is not in /Applications. It is deep inside the system folder, here:

/System/Library/CoreServices/Applications/Network Utility/app

if your top-level view of your drive does not show /System, post back with your macOS version or try

Command-Shift-> to view hidden files/folders

You want to use the "Go to Folder" option. Then enter: /System/Library/CoreServices/Applications/, and then, select the "Go" button. After doing so, you should be greeted with a new pop-up window where you will find the Network Utility app.

Note; however, since macOS Catalina, it has become highly deprecated. Most of the useful commands it had will now have to be entered in the Terminal app ... so its usefulness has been lost. Especially with folks who are not familiar with working at the Command Line level.

... but going back to your networking issue, what is the make & model of your wireless router that you are connecting this notebook to?

Your provided image also shows that the Wi-Fi channel that is running on is a DFS (Dynamic Frequency Selection) channel. These channels offer less performance than non-DFS ones. They are primarily used when your wireless router detects a nearby radar ... like near an airport. In the US, those would ones, starting at 149+ or 48-.

I think Tesserax has the key. We should ignore what Network Utility says. It appears it is not being properly supported, and that likely means it is not being properly tested.