Wifi Diagnostics get WiFi performance
When the Wi-Fi diagnostics tool finishes, the logs get saved to your desktop.

If that didn't happen or was deleted, you will have to find them from a backup or re-run the tool.



Depending on what performance you are monitoring, I would start with the two files to get a summary of the signal and noise of your Wi-Fi recording session: 


link-events.log:
link-info.log: