Any older Plone site can become obsolete - not displaying properly in the newest browsers, fixes for add-ons not available anymore etc. In such cases an upgrade of your existing Plone site to the newest Plone version is highly recommended,  offering improved content editing experience, better performance, access to the latest add-ons and more. Our team of developers has extensive knowledge and experience with Plone upgrades from versions 2.1 to the latest Plone version.

Plone core developers have spent a lot of time focusing on improvements in performance and speed to offer optimal CMS solution to the market. Particularly in Plone 4, the performance tuning was a major focus, and the results are 

Plone 4 is about twice as fast as Plone 3 was under high loads, and up to four times faster than Plone 2.5.

Plone 4 also consumes less memory, which lets you squeeze even more performance out of a given amount of hardware.

As we see in the figure below, there are big improvements for both anonymous and logged-in users.

Source: plone.org

Appart from massive performance improvements, Plone 4 also brings:

• New look - Plone 4 ships with a sleek new theme "Sunburst" that is easy to customize
• Intuitive visual editing with TinyMCE
• Improved search
• Improved handling of large files - all file and image data is now stored on the file system for improved performance and reduced resource consumption 
• Improvements to user & group management
• and much more!

See plone.org for more information about all the improvements in Plone 4.