WiFi Performance via Terminal
You can use the built in airport utility to measure SNR (Signal to Noise Ratio).  It's found in 

/System/Library/PrivateFrameworks/Apple80211.framework/Versions/A/Resources/airport

What I do is make a symlink to a directory in my path so I can call it without having to remember that long path:

ln -s /System/Library/PrivateFrameworks/Apple80211.framework/Versions/A/Resources/airport /usr/local/bin/airport


Assuming that you are connected to a WiFi network, issue the command:

airport -I

to print out information of your current connection.

$ airport -I
     agrCtlRSSI: -65
     agrExtRSSI: 0
    agrCtlNoise: -90
    agrExtNoise: 0
          state: running
        op mode: station
     lastTxRate: 243
        maxRate: 300
lastAssocStatus: 0
    802.11 auth: open
      link auth: wpa2-psk
          BSSID: 82:2a:a9:45:f3:25
           SSID: StackExchange WiFi Demo
            MCS: 14
        channel: 157,1


To calculate SNR, you take the RSSI value and subtract the Noise value.  In this case, I have an RSSI of -65dB and a Noise value of -90dB.  Calculated, that gives me 25dB.   Which is a very good signal (just barely but I am behind a reinforced concrete wall away from my AP; not bad actually considering.

SNR Guidelines


40dB+ SNR = Excellent signal
25dB to 40dB SNR = Very good signal
15dB to 25dB SNR = Low signal
10dB to 15dB SNR = Very low signal
5dB to 10dB SNR = No signal


That said..it's not all down to what your SNR is in a given location, you should also consider things like:


WiFi Saturation - excessive number of WiFi signals in the same area
Client Load - number of clients attached to a given AP
Bandwidth utilization - how much bandwidth is consumed by the clients.


If you have too many WiFi networks competing for the same bandwidth or a combination of too many users or too many bandwidth hogs (everyone watching Netflix 4K on their tablets), this will greatly affect performance beyond what your SNR can tell you.

What I prefer to do is to deploy multiple APs around the house and turn down the transmit power so they don't go very far outside the room I am trying to cover.  I personally use these PoE Access Points from Ubiquiti to cover several areas of my home with great success.