Any website that loads slowly has probably run into render blocking resources. Rendering a site means making it show up the way it is intended. If that process gets blocked in any way, the website either does not load properly or takes a very long time to do so. When the things go slowly, you lose site visitors, reputation, and rankings on Google and other major search engines.
Each of these things can considerably slow down how quickly the website appears to the casual visitor. When you get rid of render-blocking resources and improve how the browser grabs the different pieces of information, low times get smaller and smaller. Things like proper code, fewer CSS or JS files, and some in-line rendering information may help.