Images account for 80% or more of the page size on the demo and they haven’t been fully optimized. We also don’t have any form on caching on there either.
Here is a pingdom tools test https://tools.pingdom.com/#!/cMzZnu/demo.themely.com/integral/ which is quite good, though not perfect. Rarely do websites rank above 90%.
It will also always be slower on mobile devices.
We will be improving it over time but at this point it really is up to the end user to optimize images and add a caching and page optimization plugin.