fix: don't run each array expression more often than necessary

closes #14991 - at least to the extent possible

Also adjusts a comment which a) did not hint at why it's needed in the first place (was added in #14967) b) sounded like we could change that in the future, but we can't, because people will always have the ability to trigger reactivity through other means without changing the array reference

.changeset/tidy-months-rush.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: don't run each array expression more often than necessary

packages/svelte/src/internal/client/dom/blocks/each.js

@@ -136,15 +136,17 @@ export function each(node, flags, get_collection, get_key, render_fn, fallback_f
 
 	var was_empty = false;
 
-	// TODO: ideally we could use derived for runes mode but because of the ability
-	// to use a store which can be mutated, we can't do that here as mutating a store
-	// will still result in the collection array being the same from the store
+	// We wrap this in a derived to ensure possible effects created as a result are associated with the derived,
+	// rather than the each effect. Because people can mutate arrays and trigger reactivity through other means still,
+	// we need to use derived_safe_equal.
 	var each_array = derived_safe_equal(() => {
 		var collection = get_collection();
 
 		return is_array(collection) ? collection : collection == null ? [] : array_from(collection);
 	});
 
+	var first_run = true;
+
 	block(() => {
 		var array = get(each_array);
 		var length = array.length;
@@ -257,7 +259,10 @@ export function each(node, flags, get_collection, get_key, render_fn, fallback_f
 		// that a mutation occurred and it's made the collection MAYBE_DIRTY, so reading the
 		// collection again can provide consistency to the reactive graph again as the deriveds
 		// will now be `CLEAN`.
-		get(each_array);
+		if (first_run) {
+			first_run = false;
+			get(each_array);
+		}
 	});
 
 	if (hydrating) {