How do you find out why a web page is slow?

[x4294967296]

It could be a slow script, a slow internet connection, slow anti-malware software, or your computer overloaded. How do you find out which?

[Elgg]

If the webpage is slow, then nothing else would be. If everything is slow, then its something other than the webpage.
For example facebook is slow, but google isnt. Well then its something on facebooks side. Most likely overloaded servers.
If both of them are slow, it could be your computer which is slow, or your internet connection.
You can test your connection several ways. Pingplotter to check the ping times, speedtest (or similar service) for bandwidth.
If your computer is slow, then things other than your webbrowser will be slow aswell.

And antivirus/antimalware software shouldnt really make things any slower at all.