If you refactor the GetSalesAndUseTaxForState function to an async function, you can optimize it further with this: ConcurrentDictionary<string, double> ta

If you refactor the GetSalesAndUseTaxForState function to an async function, you can optimize it further with this: ConcurrentDictionary<string, double> taxMap = new ConcurrentDictionary<string, double>(); await Task.WhenAll(migrantList .SelectMany(m => m.StatesTraversed) .Distinct() .Select(async s => taxMap.TryAdd(s, await GetSalesAndUseTaxForStateAsync(s)))); migrantList.ForEach(mti => Console.WriteLine($"Total Relocation Fee: {mti.StatesTraversed.Sum(s => taxMap[s]) + mti.TotalMilesTravelled * .15}")); This gets pretty close to negligible overhead. The above ran in 0.519s, including the .5s await time of the "API" call. I just wanted to show that there's an optimized solution that is also more concise in C#.