MBP wifi speed slowed right down suddenly

By far the easiest way to cause poor performance, instability, overheating and crashing is to install ANY third-party speeder-uppers, Cleaners, Optimizers, or Virus scanners, Bit Torrent, or a VPN that you installed yourself.

¿are you running anything like that?

Hold down the Option key while you click on the Wi-Fi icon on the menubar to open up the tools for investigating and fixing Wi-Fi issues

First is "Open Wireless diagnostics", which opens the Wireless Diagnostics Assistant, but does not proceed.

There are about eight different things you can do from here, but the top-level is to choose diagnostics off the window menu, or simply click continue, to do a quick check for Gross misconfiguration or operating problems. Your Admin password will be required. If any recommendations are shown, you should consider then seriously.

Next is to hold down the Option key and click the Wi-Fi icon as before. The screen that opens shows operating parameters of your network. screenshot, transcribe, or photograph the results and post back in a reply on the forums. Looks like this older one.

so sorry I forgot to include my example:

Your 2012 model is capable of 802.11n mode only. But it has three antennas. You have a raw signal strength of -56 dB, pretty good. but your noise level at -85 dB is too high, and the patterns you were able too get do not include ANY multi-antenna patterns. You have achieved only a 122 M bits/sec transmit rate, FAR lower than what is possible for your equipment. You are carving out a 40 Hz pathway on channel 36, one of the lowest numbered (and most likely to clash) channels. My prediction is that someone else is also on "your" channel.

It does Not take a DIRECT conflict on exactly the same nominal channel to ruin your performance. Depending on channel width selected by each Router, nominal channels spill up and down the spectrum into adjacent channels.

Your selection of channel 36 spills up and down to engulf all channels from 32 to 40. Similarly another router using an 80 MHz pathway in any nominal channel nearby takes over from channels 32 to 48.

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The usual way to avoid all of this and get yourself a better channel is to make sure your Router is set to "automatic" channel selection. Then save changes and cycle to power to your router. As it wakes up, it will select the 'least busy' channel for you. It is not an accident that all debugging procedures start with "cycle the power to your Router".

if that does not provide relief, there are other tools that can be brought to bear.

AFTER cycling the power on your Router, if it has not moved to a different channel, there are two ways to see what else is in your network neighborhood.

the most visually helpful is the inexpensive utility named WiFi Explorer, and its Spectrum view looks like this:

A way that takes no additional software is to use the SCAN function inside wireless diagnostics. Launch Wireless diagnostics, and choose SCAN from its window menu. Then click on the Channel column heading to sort by channel number. its window will resemble this one:

(drag and drop on Preview to see larger and scroll)

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