Recently we experienced "flicker" on a login page. When the browser is freshed hidden containers in the UX would briefly flash on the screen before the UX rendered properly. It was worse for slow Internet connections. I found and inserted this js code just after the <head> tag to fix the problem. the "your container id here" value can be the UX container name or it can be a null value to apply broadly to the page. You can play with the numeric "delay" value (1000 below) until it works properly for your page. You want it to be the smallest number that still controls the flicker.

<!-- Anti-flicker snippet (recommended) -->
<style>.async-hide { opacity: 0 !important} </style>
<script>(function(a,s,y,n,c,h,i,d,e){s.className+=' '+y;h.start=1*new Date;
h.end=i=function(){s.className=s.className.replace(RegExp(' ?'+y),'')};
{'your container id here':true});</script>