This benchmark tests the performance differences between using a Dictionary<TKey, TValue> or an ImmutableDictionary<TKey, TValue>.
The benchmarks for adding new items show that ImmutableDictionary<TKey, TValue> performs better, in general. Although, for small sizes, Dictionary<TKey, TValue> can perform better.
On the other hand, the benchmarks for lookups show that Dictionary<TKey, TValue> performs better.