Why is my WiFi SLOW on my Mac? Troubleshoot WiFi issues on a Mac [using Wireless Diagnostics]
One of the first solutions to slow Mac Wi-Fi is to first restart your Mac’s Wi-Fi connection. This can be easily done by disconnecting it from the wireless network. To do this, click on Wi-Fi menu bar > select Turn Wi-Fi Off > Turn off Bluetooth adapters on your MacBook > Restart your MacBook and try to reconnect.