OT: why are LAMP sites slow?

[Simon Wittber]

> I notice that lots of the medium-largish sites (from hobbyist BBS's to
> sites like Slashdot, Wikipedia, etc.)  built using this approach are
> painfully slow even using seriously powerful server hardware.

Slow is such an ambiguous term. Do you mean the pages are slow to
render in a browser, or slow to be fetched from the server, or the
server is slow to respond to requests? What is slow?