OT: why are LAMP sites slow?

[Paul Rubin]

LAMP = Linux/Apache/MySQL/P{ython,erl,HP}.  Refers to the general
class of database-backed web sites built using those components.  This
being c.l.py, if you want, you can limit your interest to the case the
P stands for Python.

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.  Yet
compared to a really large site like Ebay or Hotmail (to say nothing
of Google), the traffic levels on those sites is just chickenfeed.

I wonder what the webheads here see as the bottlenecks.  Is it the
application code?  Disk bandwidth at the database side, that could be
cured with more ram caches or solid state disks?  SQL just inherently
slow?

I've only worked on one serious site of this type and it was "SAJO"
(Solaris Apache Java Oracle) rather than LAMP, but the concepts are
the same.  I just feel like something bogus has to be going on.  I
think even sites like Slashdot handle fewer TPS than a 1960's airline
reservation that ran on hardware with a fraction of the power of one
of today's laptops.

How would you go about building such a site?  Is LAMP really the right
approach?