There’s no question that plugins are one of the strong benefits of using WordPress as a CMS.
It’s also one of its biggest challenge. Due to the fact that, in my experience, WordPress.org has never once updated their mess of a plugin directory, it can be difficult to find the best plugin. Nearly everyone wants a SEO and caching plugin on their site, but it’s hard to determine the best one (or ones) without some fairly detailed research.
Unfortunately that often leads people to just grabbing and installing a mass of plugins, or simply choosing poor plugins. All of this can have a pretty terrible effect on a site’s load time.
This article does some testing on a popular group of plugins, highlighting how much these plugins can impair a WordPress site from running smoothly.
I’ve found this to be a challenging issue to discuss with clients. They will ask me to speed up their site, and while you can go ahead and squeeze a lot of optimization from caching, image compression, efficient themes, etc, it can often come down to one plugin that causes all of the slowdown issues.