Basic benchmarks around implicit/explicit contra/covariant casting.

ImplicitCastingBenchmarks.cs

using BenchmarkDotNet.Attributes;

namespace ClassLibrary1
{
	public class ImplicitCastingBenchmarks
	{
		readonly private IInterface<string> _sut = new Implementation();
		readonly private IInterface<object> _obj;
		public ImplicitCastingBenchmarks()
		{
			_obj = _sut;
		}

		[Benchmark(Baseline = true)]
		public void Direct() => Specific(_sut);

		[Benchmark]
		public void Implicit() => General(_sut);

		[Benchmark]
		public void Explicit() => Specific((IInterface<string>)_obj);


		private static void Specific(IInterface<string> parameter) => parameter.ToString();

		private static void General(IInterface<object> parameter) => parameter.ToString();

		private interface IInterface<out T>
		{
			T Get();
		}

		private class Implementation : IInterface<string>
		{
			public string Get() => string.Empty;
		}
	}
}

Reference: https://www.danielcrabtree.com/blog/191/casting-to-ienumerable-t-is-two-orders-of-magnitude-slower

// * Summary *

BenchmarkDotNet=v0.10.3.0, OS=Microsoft Windows NT 6.2.9200.0
Processor=Intel(R) Core(TM) i7-4820K CPU 3.70GHz, ProcessorCount=8
Frequency=3613280 Hz, Resolution=276.7569 ns, Timer=TSC
  [Host]     : Clr 4.0.30319.42000, 32bit LegacyJIT-v4.6.1637.0
  DefaultJob : Clr 4.0.30319.42000, 32bit LegacyJIT-v4.6.1637.0


   Method |       Mean |    StdDev | Scaled | Scaled-StdDev |
--------- |----------- |---------- |------- |-------------- |
   Direct | 11.6787 ns | 0.1127 ns |   1.00 |          0.00 |
 Implicit | 11.7160 ns | 0.0766 ns |   1.00 |          0.01 |
 Explicit | 79.9238 ns | 0.6084 ns |   6.84 |          0.08 |

// ***** BenchmarkRunner: End *****