Problem with background colour during site navigation

I have just started putting together a new site based on the FlatIron template. All I have so far is a home page with 5 galleries, about and contact pages. I have set the background colour for all sections of the site to duck-egg blue (hsl(180, 20%, 85%). All good so far.

However, when navigating between pages, there is a very clunky process of page loading, and a white page flashes up and is on screen long enough to be irritating (actually even a flash of white would be irritating). I have noticed that I often need to click navigation items twice before I actually move to the target page. The first click causes the current page to disappear, I get a blank duck egg blue page, then a flash of white page, and am returned to the page I was on. On the second click all the same happens but I do end up on the required page. It doesn't happen all the time, but something is surely wrong here?

Any ideas welcome  (I'm pretty inexperienced at all this!)

Site URL: https://spinach-jaguar-ysxj.squarespace.com/  pass:threeislands

Having changed the site to password protected so that it could be viewed from the link, I have found that the site runs smoothly when not being edited/designed.  

So problem appear on edit mode only? If yes, you will need to contact SS Customer Care, they can check backend to troubleshoot the problem