This is weili，
my website :www.kigrumi.com
When I open my website,Incomplete loading will always appear briefly, as shown bellow.
Return to normal after 2 seconds.
My Theme buy from a third party, I want to know if it’s Theme problem or something else
It's definitely caused/should be mitigated by the theme.
Looks like the theme has a loading animation to hide this, but it was disabled in Layout Hub settings? Also, I guess it was still looking ugly after loading animation turned off.
Though It's possible to reduce the time this unformatted menu data is shown by slightly modifying the megamenu HTML.
No, it's not quite like this.
First of all -- it's theme developer's responsibility to ensure that site looks reasonable even on slow connections, not yours.