First, here is a pointer to a somewhat out of date performance comparition dated December 2004 and evaluates Plone 2.0. http://www.ifpeople.net/learn/resources/downloads/PlonePerformance.pdf
Even if the Plone version is not up to date anymore, the effects than can be achieved by caching are interesting.
Plone 2.5 and 3 can also take advantage of CacheFu, a Product that optimizes your Plone for cacheability.
Recent efforts to speed up Plone have been undertaken during the Plone Performance Sprint in Copenhagen. The page also describes some tools that can be used for benchmarking.
The traditional ways for speeding up Plone usually include putting Apache and Squid in front of it. Recently there are some new kids on the block, namely NginX and Varnish which are definitely worth a try for very high performance. Some videos on an implementation by lovely systems and the slides.
Of course scripts in your site which have a long running time can also be responsible for slowness. Why not cache them? Or as Joel Burton says: Plone Does a lot - This is Good - This can be Bad. There are very extensive slides from the talk he held at the 2007 Plone Conference names High Performance Plone.
Developers should take a look at the performance section of the Plone Developer Reference to avoide common mistakes.