A performance comparison between ToArray() vs ToImmutableList().

ToArrayVsToImmutableList.cs

#r ".\System.Collections.Immutable"

using System.Collections.Immutable;
using System.Diagnostics;
using System.Linq;
using static System.Console;

class Person
{
    public int Id { get; set; }
    public string Firstname { get; set; }
    public string Lastname { get; set; }
}

var data = Enumerable.Range(0, 100000).Select(i => new Person { Id = i, Firstname = $"Vorname {i}", Lastname = $"Nachname {i}" }).ToList();

long TestToArray()
  => Enumerable.Range(0, 10000)
  .Select(_ =>
  {
      var sw = new Stopwatch();
      sw.Start();
      data.ToArray();
      sw.Stop();
      return sw.ElapsedMilliseconds;
  })
  .Sum();
  
long TestToImmutableList()
  => Enumerable.Range(0, 10000)
  .Select(_ =>
  {

      var sw = new Stopwatch();
      sw.Start();
      data.ToImmutableList();
      sw.Stop();
      return sw.ElapsedMilliseconds;
  })
  .Sum();
  
WriteLine(TestToArray());
WriteLine(TestToImmutableList());