This is the first I’ve heard of any slowdown in the builder lately. Sorry you’re experiencing that. Can you write into support with a link to your app if you haven’t already?
One thing to try right off the bat is if you use the Lottie component, turn off all animations in the builder. Some folks fill their app with Lottie animations and they’re all looping all the time in the builder and really chewing up performance. Most of the time when I see someone’s app slow in builder, this is the cause.