You got a really weird number at String.IndexOf. Here are my results:

Elapsed Time for [String.Replace]: 16,8695848 sec
Elapsed Time for [String.IndexOf]: 2,0153508 sec
Elapsed Time for [String.SubString]: 11,7445885 sec
Elapsed Time for [String.Remove]: 10,1579051 sec

.NET 6 is 1.7x up to 2.9x faster. I think these numbers are pretty consistent.