I've used many a government website in the Navy, and they were almost invariably bad, but it had nothing to do with React per se.

A very slow website I can think of had something like 200 GET requests required to load the landing page, and it used Liferay with Material Design Bootstrap. That was closer to the "style at the time". React is the style of this time, but you can write very slow websites in anything, I'm convinced.