A common misconception about WP Super Cache is that is works by increasing how much is cached on website visitors computers. It really works by caching was is on your website’s server.
Without a cache your website server combines a series of PHP files to generate the HTML of your website. It may also call a series of scripts and css files depending on what plugins are installed. What often makes a website slow is not how big the files are being downloaded, but how many there are. Each time the server loads a file it takes time no matter how small it is.
Note that when you are working on your website you should disable WP super cache and then re enable it when you are finished. If you do not disable the plugin you will not see the updates to your website because you will download the cached (minified) version.