As other respondents have noted, new features are usually slowing down a website – but only under specific circumstances.
Adding new features (through plugins or additional snippets) leads to additional code that should be executed and processed in order to provide the new functionality.
Some of those features may be tiny, and add insignificant load to your site (talking about nanoseconds). Others are far more complicated, and could increase the load time with anywhere from 100ms to 1–2 seconds. Good examples for those are multilingual plugins, eCommerce solutions, LMS platforms and others on top of WordPress as the database grows and requires more time to sift through data and provide the correct results.
Often times, those features are essential to a business. The good news is that there are successful strategies for optimizing a WordPress website that allow for compressing images, combining and minifying scripts and styles in order to reduce the HTTP requests and the size of a page, caching resources in several different layers, using a Content Delivery Network and more.
A common misconception in the WordPress space is that the number of plugins is directly related to the load time of the website. In fact one plugin may be complex and heavy enough to outnumber dozens of small and simple plugins that are only executed in certain cases (and on specific screens).