We were doing some unnecessary work on these fast paths:

• We were calling ConfigureFormatR/O even though the work they were doing was never used. Removed the calls and then inlined the code into the remaining call sites.
• Removed unnecessary arguments.
• Removed a virtual dispatch calling to GregorianCalendar when we could instead just go straight to the helper on DateTime.
• Reduced number of cases in switch statement.
• I also noticed and cleaned up an unnecessary delegate involved in Hebrew number parsing.

private string _r = new DateTime(1955, 11, 5, 6, 0, 0, DateTimeKind.Utc).ToString("r", CultureInfo.InvariantCulture);
private string _o = new DateTime(1955, 11, 5, 6, 0, 0, DateTimeKind.Utc).ToString("o", CultureInfo.InvariantCulture);

[Benchmark] public DateTimeOffset ParseR() => DateTimeOffset.ParseExact(_r, "r", CultureInfo.InvariantCulture);
[Benchmark] public DateTimeOffset ParseO() => DateTimeOffset.ParseExact(_o, "o", CultureInfo.InvariantCulture);