Choosing Performance
It’s not because of any sort of technical limitations. No, if a website is slow it’s because performance was not prioritized. It’s because when push came to shove, time and resources were spent on other features of a site and not on making sure that site loads quickly.
This goes back to what many have been stating as of late: performance is a cultural problem.
Tim Kadlec in Choosing Performance on why the web is slow.