Commercial WordPress themes generally include a large suite of scripts and styles, along with a comprehensive set of features in order to make them pop.
Most premium themes rely on a number of theme options, various widgets, eventually some sliders, content builders, galleries, social media integrations and the like.
Light, Simple Commercial Themes May Work Faster
Most of the commercial themes – especially the multipurpose ones – are simply trying to solve a large set of problems for different industries and types of websites. That’s what causes delays, instability, and possible security issues.
As a rule of thumb, the free themes hosted on WordPress.org only care about the presentation layer – usually comprised of markup and CSS. They’re fairly lightweight and simple, going through a rough reviewing process. The general principle of themes is that they should only serve for presentation of the standard blog/CMS content. Anything else should be distributed as a separate plugin – in order to keep things separated and avoid a theme lockdown whenever you decide to redesign your website.