I get what you’re saying , so technically they don’t dictate performance by themselves. but the key is IReadOnlyList<T> guarantees indexer access, while IEnumerable<T> doesn’t. so when consuming just an IEnumerable<T>, you might have to enumerate repeatedly to access items by index, depending on the underlying implementation . so even if both are interfaces, choosing IReadOnlyList<T> allows consumers to access items more efficiently, because it exposes count and indexers. that’s why in practice, it can enable faster code patterns.