Plone's uber-trainer and developer extraordinaire, Joel Burton, gave a wonderful presentation today on ways to make Plone run fast. Really, really fast.
The Plone Web content management system has long had a somewhat deserved reputation for being slow. However, that's truly only because Plone is optimized out-of-the-box for development, and people don't always figure out that reconfiguration is needed to get great performance in a deployment situation.
Sites like Novell.com and CIA.gov clearly show that Plone can perform extremely well, if only a bit of care and attention is paid by the developer.
You can download the presentation here.