If I have extensions "installed but disabled" in my extension manager (as opposed to "completely uninstalled"), do these disabled extensions slow down web site performance for web site visitors?
If yes, why?
Actually it does slow the site down in the slightest by having them disabled rather than uninstalled, as when loading a plugin, it checks the ones that are enabled and runs them. If you have 100 plugins for example and 50 of them are enabled, then the
where() clause will have to sift through the 50 disabled ones too. Its not a speed that you will notice that much (depending on how many extensions you have), but it would be just best to uninstall them.
When viewing the extension manager in the backend, you will actually notice the speed difference.
If you are worried about the speed of your website because of disabled extensions - then you are worrying about the wrong thing... Get a decent webhost and the milliseconds your site consumes because of disabled plugins will be a non-issue.
Also just cause you have disabled it in Joomla doesn't mean it cannot be accessed directly by URL, and any security vulnerabilities it may have may still be exploited.
Lastly, just because a plugin is disabled, doesnt mean some plugins do not fire/run, some plugins will be invoked and they will check themselves if they are allowed to run or not!