When it comes to first impressions, it can be said a website is made or broken by them. A modern website that relies heavily on scripting is likely self inflicting a performance hit. This hit can be caused by the generic advice given for speeding up a website, ironically. Many web speed tests give an immediate failing grade for not using a content delivery network (CDN) for every single one of your assets. But in the case of critical scripts and styles, this can be the exact wrong advice.
Critical scripts and style are the assets that must load prior to effectively loading content. For instance, a site might use Modernizr to sniff for features to further load content or other scripts. It might use jQuery to create DOM elements and place them on the page. It also might use @media queries to alter, import or change the layout of the page. If any of these situations exist, a new or returning user will have to wait for that file to load on the page prior to being able to view any content. I’m not going to argue if a site needs to be doing those things first, but if it is, then it’s going to cause a ‘hiccup’ for first time rendering. A cached asset may negate this on further loads and that’s why caching and cache control is a good thing to use. But this does nothing to help a first impression.