Yeah. What stevo_ said.
The intent of the code cannot be transmitted to people who don't know the RegEx syntax, but could be lost to the original author, who might have to deal w/ RegEx's that one time 6 months ago.
Actually, they're not that slow at all - remember that if you declare a regular expression with the RegexOptions.Compiled option the expression is compiled to an assembly. While this adds a small amount of overhead on first use, any subsequent use is very fast
As an example, the following regular expression will match all repeated words such as in
Paris in the
Regex regex = new Regex(@\b(?<word>\w+)\s+(\k<word>)\b",
RegexOptions.Compiled | RegexOptions.IgnoreCase);
Now think about how much harder it would be to do the same thing without regular expressions.