I’m guessing this is a theme you’ve customized, not a brand-new, unmodified Sage install? If that’s the case it’s very difficult for us guess at what might be causing the issue.
A good place to start would be to go into the Safari network inspector and look to see if any of your assets are taking a particularly long time to load, which might point you in the direction of a problem. For instance you could be attempting to load some kind of remote asset and Safari allows for a much longer wait before it times out than other browsers.