One possible issue may be the result of your Navigation Menu width and styles.
See here:
The screensize i have set is 1366px wide, which is still one of the most common screen sizes.
I then switch the font weight from the 400 default to 900 weight from your customizer settings.
Just that tiny change in size causes the menu to wrap to two lines.
On slower networks there is a chance that the additional font styles load later which causes the layout shift.
I would recommend that you reduce the risk of this occurring by reducing the width the menu occupies.
Some options for doing so:
1. removing top level menu items.
2. reduce font size / font weight
3. reduce menu item width in Customizer > Layout > Primary nav.
4. choose a narrower font