All page builder plugins add some extra weight vs just using the native WordPress editor, so a page builder will never be as lightweight as just using the native editor. However, as far as page builder plugins go, Beaver Builder is one of the more lightweight and performance-friendly plugins that you'll find. I've tested all of the popular page builder plugins multiple times and Beaver Builder routinely has the smallest page size and the fewest number of HTTP requests. So, for a page builder, I would say that Beaver Builder is very fast.