This is an issue that has plagued us for quite some time. It’s mainly due to the way some browsers work. For example, it wasn’t too long ago when chrome was particularly susceptible to this issue. That was due to the page being displayed before all the js and css elements have been fully loaded. In contrast, other browsers would wait for everything to be loaded before displaying the page. If the page IS displayed before everything is loaded, you will momentarily see a blank white page which we see as a flash.
The issue may also be as a result of a particularly large hero image on the page which takes a little time to load. However, this isn’t usually a problem because things like page colour are usually part of the CSS that gets loaded first. I would try playing with some of the export options in Blocs to see if that improves things. For example, enabling and disabling lazy load. Also remove any pre-loaders that you may have installed.