Basic benchmarks on casting, including with access from a dictionary.

CastingBenchmarks.cs

using System;
using System.Collections.Concurrent;
using System.Collections.Generic;
using BenchmarkDotNet.Attributes;

namespace ClassLibrary1
{
	public class CastingBenchmarks
	{
		readonly private static Type Key = typeof(int);

		readonly private IDictionary<Type, IInterface> _items = new ConcurrentDictionary<Type, IInterface>();
		readonly private IInterface _implementation = new Implementation();

		public CastingBenchmarks()
		{
			_items.Add(Key, _implementation);
		}

		[Benchmark]
		public IInterface Access()
		{
			IInterface result;
			return _items.TryGetValue(Key, out result) ? result : null;
		}

		[Benchmark]
		public IInterface<int> AccessCast()
		{
			IInterface result;
			return _items.TryGetValue(Key, out result) ? (IInterface<int>) result : null;
		}

		[Benchmark]
		public IInterface Instance() => _implementation;

		[Benchmark]
		public IInterface<int> InstanceCast() => (IInterface<int>) _implementation;

		public interface IInterface<in T>
		{
			void Execute(T parameter);
		}

		public interface IInterface
		{
			void Execute(object parameter);
		}

		private class Implementation : IInterface<int>, IInterface
		{
			public void Execute(int parameter) => parameter.ToString();

			public void Execute(object parameter) => Execute((int) parameter);
		}
	}
}

Results:

// * 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 |    StdErr |    StdDev |     Median |
------------- |----------- |---------- |---------- |----------- |
       Access | 21.5227 ns | 0.0318 ns | 0.1232 ns | 21.5749 ns |
   AccessCast | 86.1340 ns | 0.0804 ns | 0.2897 ns | 86.1233 ns |
     Instance |  0.0091 ns | 0.0028 ns | 0.0108 ns |  0.0000 ns |
 InstanceCast | 63.0500 ns | 0.0762 ns | 0.2850 ns | 62.9240 ns |