I mean it's basically the same thing as you do with React. You create lens functions more or less to handle get/set over the state object. Solid's setState has a lot more capability than React's handling nested updates with an ImmutableJS or Immer inspired API. The clever part here is immutable libraries already have API patterns to describe mutation so I just apply them here. So instead of spreading everything and cloning you keep the mutation locality for performance but keep the control of immutability.

Also don't be worried about copying the array that much. I'm doing that in the JS Framework Benchmark and it's still about the fastest with 10k items. The real key is identifying where you don't need to update the list at all just the items in it. That being said you can do an update without copying the array like this:

setState('list', state.list.length, newItem);

Granularity of performance isn't a silver bullet. Being fine-grained doesn't necessarily make everything faster (especially creation). The power is that granularity is arbitrary so it can be maximized to the type of situation, completely independent of component structure.