I looked at the diff, but I’m not entirely sure I get it — why does this change improve the framerate so much?

Reanimated is basically a babel plugin that converts your code to multi-threaded concurrent JS. This means that when you have a worklet, it has to serialise the closure to the UI thread which can be really slow when you’re closing over more than you realise. Although in this case I think it’s a bug

This doesn't have to do anything to do with serialization. Worklets are only "serialized" when created, so in this instance only when headerMode changes, which I assume isn't frequent. Serialization only happens once for a given object and then it is cached, so withSpring is only serialized once

What this change does it is simply skips calling `withSpring(0, {})` in favor of executing it only once when mutable is created and caching that value. withSpring returns an animation object which gets cached in a mutable in this case.

You should be able to verify that by leaving reference to withSpring inside that worklet but just not calling it. In that case this should not degrade performance. I believe it is simply the runtime cost of withSpring call that needs to be optimized