From b2evolution manual
This manual page is outdated. NEW PAGE: Performance Optimization in b2evolution v5+.
There are several things you can do to optimize the performance of your system.
 Fast Hosting
Once you have a reasonably fast web host, the one single biggest performance enhancing technology is caching.
b2evolution can take advantage of several different caching mechanisms. Some are enabled by default, some need manual activation or installation.
Please refer to the Caching and cache levels section for more information on all these caches.
 Plugins & widgets
While some plugins are actually designed to improve performance through various tricks, some other plugins are programmed without any consideration given to performance. Do a test deactivating all your plugins and see if it makes a difference.
Even built-in widgets can hinder performance if you use too many in your sidebar for example. This can usually be resolved by enabling caching though.
 Apache tuning
On apache2, make sure you enable mod_deflate so that the HTML being sent back to the requesting browser is as small as possible.
This is the second most significant performance improvement you can get after enabling caching.
mod_deflate can divide the amount of data being sent by 3 to 5! Even on high bandwidth internet connections, the difference between sending 20 and 5 Kilo Bytes of data can be huge! This is because it's not only about bandwidth but also about latency times and TCP window sizes. Small files can really go through exponentially faster!
Without an Expires: header, the minimum that will happen is that the browser will send a request to see if the file has been modified since it last got it. It may receive a "304 Not Modified", but that still takes visible time, for example on transcontinental connections.
With the Expires: header, the browser will not try to ask if the file has changed until it expires. (Unless, of course, somebody hits Refresh!)
Once the module is enabled, you still need to configure it. This can be done in .htaccess if you don't have access to apache2.conf. We suggest something like this:
Be careful not to enable Expires: for text/html if you don't do exactly what you are doing. Doing this may lead a user to seeing data cached from a previous user when different users share a browser (or when logging in / logging out).
 More Apache tuning
Here are interesting resources about Apache tuning for PHP: