Try different browsers (Chrome, Firefox etc) and if possible different computers on your network. If it doesnt change between computers, it's your internet connection.
If its only your computer, but across all browsers, you will need to do some further diagnostics (e.g. network card drivers etc - I can't advise here really) and if its fixed with a diff browser - use it.
you could try some ping tests to various websites. If download tests and pings are OK it may be your dns server. It could also be your router/modem incorrectly configured.
Good luck diagnosing it