This is not a known issue, but it is indeed theoretically possible if a page is too heavy or if there is something that slows down the browser before the Kendo UI widgets are initialized.
Please check a similar thread in our forum:
Optimising the page or hiding the elements are the only suggestions in this scenario, as the Kendo UI team has no control over it.
If these approaches do not help, I can make a suggestion to check the nested HTML elements on the page.
Depending on the scenario, there are some combinations of nested elements which will take more time to properly adjust the layout which can cause the flash of unstyled content. Please start removing nested elements from the page to determine which one is causing the issue.
Still, if additional assistance is needed, please send a fully runnable example so I can test it and check if additional optimisations can be made.