Problem with bootstrap mobile menu, what's the problem?

Question

css

There is a standard menu on bootstrap 4, which on the mobile version is converted to a hamburger, which in turn displays a dropdown-menu on click, everything is default. No animations or effects.
The problem occurs when opening the site on a mobile device. A multi-level menu, when opened, is first displayed without styles, then it is immediately styled, but this is visually noticeable. With what it can be connected? Thanks in advance.

Answer 1

See how the css file is loaded in Devtools in the Network tab, you can set Slow 3G)
Most likely, the file weighs a lot ...

Answer 2

The standard excuse, which is correct 95% of the time: cached?
Try incognito on mobile.
If not, then enable remote debug for chrome.
https://developers.google.com/web/tools/chrome-dev...