Протестил):
System.IO.Path.GetExtension: ext=.txt milSec=00:00:00.0000016
Substring: ext=.txt milSec=00:00:00.0000019
System.IO.FileInfo: ext=.txt milSec=00:00:00.0000445
Split: ext=txt milSec=00:00:00.0000009
System.Text.RegularExpressions.Regex: ext=txt milSec=00:00:00.0004294
=)