OK, this is a weird one. For reference, I've dug into CSS issues, and JavaScript issues, and I can't seem to find why this is happening.

When you first go to the staging site, part of the secondary menu appears:

After about 10s, the remainder of the header appears:

I'm thinking it is NOT something Divi related since the secondary menu is partially showing. However, it could be some simple setting in Divi I'm unfamiliar with.

I'm suspecting there's a CSS animation somewhere, as you can see from the Animations section of the performance tab below:

Anybody have any insights or suggestions as to what else I could try?