One of most important factors in the site's overall speed is the amount of data the server needs to handle to generate your page. Each Joomla! extension you add to your website requires system resources, database and disk space to operate. Obviously, adding extensions - big or small - will increase the overall size of your website and thus make it slower.
So, it is easy to understand why is so important to make sure that you have installed only extensions that you’re actually using. During the building of the site we often test different plugins, components, and modules to pick up the one which best fits our needs. Also as the time passes, some of the extensions initially used might become obsolete. Also during migrations we are often ending up with remnant of the extensions written for previous versions of our beloved CMS.
We are all humans ;) and we often forget to remove these extensions, and forget that they act as a ballast, slowing down - and even making vulnerable - our site. It’s a great idea to regularly do a clean-up of such extensions just to make sure that your site is slim and fit, not bloated by unnecessary things.
Also it is adviseable, that when you search for new extensions to use on your site check one important thing - often overlooked - about them: their impact on overall site speed. There are two usual tactics here. First is to check other users opinions, reviews in the extension's site, in the JED or in official Joomla forums about the impact on the site speed, or, if you have a shortlist, do a simple check: measure your site's performance before and after you install the new extension. This can give you an idea about what to expect when you enter in production mode the newly selected extension. None of these tactics are foul-proof, but can reveal the key weaknesses of the extension, and you can have an idea if the extension is poorly written - so will be a resource hog - or have just the acceptable impact on your site's overall speed.
