Are websites slow or is it just one network?
Most websites are served by a handful of big networks. If only some sites are slow or won't load, one of those networks may be badly routed from your provider. Weak Wi-Fi or a busy line slows every network alike, so comparing them tells you which it is.
Website network check
Takes about ten seconds. Turn off any VPN first, or it tests the VPN's route instead of yours.
One service down for everyone? Try Is it down?. Everything slow? Run the Lag Check to see if it's Wi-Fi or lag under load.