Dev4Press has an interesting post that contains performance benchmark numbers that show just how much of an impact certain plugins have on loading times within WordPress. Amongst the 35 tested plugins are bbPress, W3 Total Cache, WooCommerce and a few of the plugins developed by Millan. I as well as many others were shocked to see bbPress with such poor numbers thanks to it loading everything on every page load instead of only what it needs. Keep in mind that it’s not about how many plugins you have installed on your website, but which ones. It only takes one poorly coded plugin to cause you grief.
For some additional reading on how to optimize plugin loading, please see this tutorial by Millan.
Hat tip via WPCandy.com.