Diagnosing problems

A short list that finds most faults.

Work up the layers

Check the link, then an address, then a name, then the service. ping tests reachability, traceroute shows the path, and nslookup or dig test DNS specifically.

Try the IP address directly: if that works and the name does not, it is DNS.
Try another device on the same network to separate device from network.

In the browser

The network tab shows every request, its status and its timing. A slow page is usually one blocking request rather than the connection being generally slow.