Generally the lighter the racket the easier/faster it will be to maneuver, wasn’t the latter the topic of this thread?
Wouldn’t use anything lighter than 4u either but if you are talking about the fastest rackets it would be wrong not to talk about the super light ones.
Seems there is a market for these though. I know that some of our club’s mixed doubles ladies use rackets around the 6u category (but doesn’t give them any chance at all once they are stuck in the rear court).
When it comes to professionals Li Junhui, arguably on of the most dangerous rear court players over the last few years, has been using the N7 II Light which comes at 75-79g (or a reapint as many on this forum were suspecting).
But some users here and over on the badminton subreddit have been looking for super light rackets 5u and upwards due to fatigueing quickly when using 3u or 4u, so why not if it works out for them.