It really depends on the plugin, but yes WordPress plugins can affect your site speed if they’re not coded well. However most well-coded WordPress plugins will NOT slow down your website especially if you’re using proper caching and following our WordPress performance best practices.
WordPress plugins that run exclusively on the admin area like WP Mail SMTP, Uncanny Automator, etc will never slow down your website because they don’t load on the frontend.
But when a plugin loads assets like images, scripts, and other files as your website loads for users (frontend), then they can impact your overall website speed. This is why choosing the right WordPress plugins is important, and the ones that we recommend in our list are performance optimized.
For more details, see our guide on how WordPress plugins affect your site’s load time.