The general issue with many so called “multi-purpose” themes from the performance point of view is that, by design, they focus on flexibility first.
A multi-purpose theme usually would be created and marketed with one single promise: to enable people who don’t necessarily know how to build a website to build a website.
Thus, flexibility and usability for the “builder” always come first. Accessibility and performance for the actual website visitor come second, or third—even if some of those themes provide their own “performance” features.
Caching plugins can make a page generated by a multi-purpose theme faster, but they can only do so much. Loading times will improve relative to size and speed of the non-optimized page.
For example, if a page would take 6 seconds to load without any optimization, a caching plugin may improve that by 100%, or even more. Yet, with a 100% improvement said page would still take 3 seconds to load—and no caching plugin could do anything about that.