The line that matters is:

  configBuilder.AddJsonFile(f, true, true); 

  var configBuilder = new ConfigurationBuilder();
  
  var configFiles = new[]
  {
      Path.Combine(AppDomain.CurrentDomain.BaseDirectory, "appsettings.json"),
      // the files below do not exist, not even the directory - just a fallback
      Path.Combine(Environment.GetFolderPath(UserProfile, DoNotVerify), ".tester/appsettings.json"),
      Path.Combine(Environment.GetFolderPath(ApplicationData, DoNotVerify), "tester/appsettings.json"),
  };
  
  foreach (var f in configFiles)
  {
      // using the line below works fast like expected
      // configBuilder.AddJsonFile(f, true, false); 
      
      // HUGE slowdown of app startup with second parameter set to true. Why?
      configBuilder.AddJsonFile(f, true, true); 
  }
  
  var config = configBuilder.Build();
  Console.WriteLine("tester");