This is definitely a notable difference between the form designer and classic forms. Classic forms waits to render the page until all initial field rules and lookup completes. This may look fine for simple forms, but in complex forms or if there are any delays in the lookups it dramatically increases the first paint of the form.

In the new form designer we explicitly do not wait for anything before painting the page which in your case leads to some blips in show/hide logic, but more commonly is a better user experience. 

I think a middle ground we could do is to better establish the initial state of fields from field rules based on the form's static (default) values. So in your case the field would default hidden because it relies on the value to be "Show" but the static value is undefined. We would of course not touch on load lookup rules and any field rules affected by lookup rules would not see any changes.

You could control could do now though is to control what is shown yourself with CSS or JS. If all of your fields should be hidden by default you can hide them with CSS and remove the class in JS. Another thing I have done before is use custom html in the form description to show a loading bar and wait for a series of lookups to complete before populating the form.

An example hidden CSS class that doesn't hide the element in the designer is:
 

.hidden {
  display: none;
}
.hidden:has(.selectors-display) {
  display: block;
}
.hidden:has(.selectors-display):after {
  content: "hidden";
  position: absolute;
  top: 0;
  right: 0;
  border: 1px dashed gray;
  background: lightgray;
  padding: .5rem;
  border-radius: 4px;
}