As a developer I'm not too happy about the trend I've seen in the past couple years of 'monolithic' WordPress themes with tons of JS and other code, because making even simple changes to them can get extremely complicated. I think they caught on for two reasons (1) they often have interactive 'builders' that come with widgets you can drag and drop (contact forms, slideshows, etc.) and (2) The whole design trend of things that change as you scroll--'parallax' elements and dynamically-resizing headers, etc.

I've always made my own themes, and even those can be fairly slow without the standard caching plugins. Optimizing Craft CMS sites was a breeze by comparison.