The amount of data transferred is only part of the issue. Site publishing tools like WordPress often regenerate every page each time it is accessed, even if that page rarely changes. In these instances, even the most optimized pages can feel slow as they take seconds to generate.
Many tools offer the ability to enable caching, a system that stores page content after it is created. WordPress has various caching plugins that keep page content around between loads. When content changes, such as by adding a post or leaving a comment, the cache is updated and the new page content left in place.
Even with smaller pages and caches, sites can feel slow if they take time to download. Audiences on the internet are global, and transmitting pages and images to remote locations results in slow speeds and broken downloads.
This is where a content delivery network can help. CDNs sit between websites and their viewers, and act like caches on a global scale. Say a website is hosted in Dallas, but is viewed by someone in London. Web requests travel from the reader, through the CDN and finally reach the website. If the CDN has a server near London, that server downloads and stores a copy of the website when it is first accessed. Subsequent site access retrieves the version stored in London, thus saving the longer and slower download all the way from Dallas.