Reduced memory footprint
Plone 4 brings several behind-the-scenes improvements that work together to reduce its memory footprint and let you squeeze more performance out of your hardware. On average, Plone 4 uses about 20% less RAM than Plone 3. Even better, Plone's memory consumption no longer grows over time but remains constant after initial caches are filled up.
We achieve these impressive improvements through a variety of approaches. First, Plone 4 handles translation files in a smarter and more memory-efficient way. Plone 4 also uses Python 2.6, which improves memory handling and garbage collection and allows Python to give unused memory back to the operating system.
The other major feature we've added that reduces the memory footprint is a new version of the Zope Object Database (ZODB) that includes support for storing and serving large files (aka "BLOBs") directly from the file system. This means that Plone uses a lot less memory when serving, receiving and storing files and images.