Do plugins reduce performance?

If you add plugins that are only for the Admin interface, they will have no effect on your regular visitors. If, however, you add plugins that modify the content of your posts in one way or another, the more “filters” they go through will involve some resources to be used.

Additionally, if the plugin utilizes the database in an inefficient way you’re going to notice it.

Most plugins are very small and do things very quickly and you shouldn’t notice much of a difference. But there are a handful of plugins that are very ellaborate and could potentially slow things down.

If you find yourself in that boat, there are way of storing modified content in the database such that you don’t keep applying the modifications every time, but that’s something you should look into when you find your site slows to a crawl.

Regards

Depends on the plugin but the simple answer is “quite likely.” ??

But seriously, if you’re using plugins to, for instance, grab the most recent comments or perform other database accesses, that will obviously have an effect on performance. Using Textile or Markdown takes longer to process than rote HTML, so using those will cause more processing per request. All plugins add overhead to the processing of data, even little bitty seemingly useless ones. Some use more than others.

EDIT: As Cypher said, plugins that are meant to affect the admin interface will have negligable effect on your users but may have some, simply due to the way the plugin architecture is structured. Any time there’s a request every plugin you have activated must be loaded and executed. In the case of admin plugins, they may not do much besides install filters to be run that won’t run for user pages.

Do plugins reduce performance?

Yes, but only by a few milliseconds. It’s barely noticeable.