More benchmarks on https://www.youtube.com/watch?v=jmmz1cInNow&ab_channel=NickChapsas

ForBenchmark.cs

using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;

namespace ForBenchmark
{

    [MemoryDiagnoser(false)]
    public class Program
    {
        [Params(10, 1000)]
        public int MAX {  get; set; }

        [Benchmark(Baseline = true)]
        public void ClassicFor()
        {
            for(int i = 0; i < MAX; i++)
            {
                DoSomething(i);
            }
        }

        [Benchmark]
        public void ForEachCustomEnumerator()
        {
            foreach (var i in 1..MAX)
            {
                DoSomething(i);
            }
        }

        [Benchmark]
        public void EnumerableRange()
        {
            foreach (var i in Enumerable.Range(1, MAX))
            {
                DoSomething(i);
            }
        }

        [Benchmark]
        public void EnumerableRangeListForEach()
        {
            Enumerable.Range(1, MAX).ToList().ForEach(i => DoSomething(i));
        }

        void DoSomething(int i) { }

        static void Main(string[] args)
        {
            var summary = BenchmarkRunner.Run(typeof(Program).Assembly);
        }
    }

    public static class Extensions
    {
        public static CustomIntEnumerator GetEnumerator(this Range range)
            => new CustomIntEnumerator(range);
    }

    public ref struct CustomIntEnumerator
    {
        private int _current;
        private readonly int _end;
        public CustomIntEnumerator(Range range)
        {
            if (range.End.IsFromEnd)
                throw new NotSupportedException();
            _current = range.Start.Value - 1;
            _end = range.End.Value;
        }

        public int Current => _current;
        public bool MoveNext()
        {
            _current++;
            return _current <= _end;
        }
    }

}