I stumbled upon this by chance : the regex in Hyena.StringUtil.CamelCaseToUnderCase is re-created for each call.
I'll attach a patch to make the regex static

Here the relevant "mono --profile" output before the patch :
Time(ms)      Count P/call(ms) Method name
########################
 1628.804     146   11.156   System.Text.RegularExpressions.Regex::Split(string,string)
  Callers (with count) that contribute at least for 1%:
         146  100 % Hyena.StringUtil::CamelCaseToUnderCase(string,char)


After the patch :
########################
 279.055     144    1.938   System.Text.RegularExpressions.Regex::Split(string)
  Callers (with count) that contribute at least for 1%:
         144  100 % Hyena.StringUtil::CamelCaseToUnderCase(string,char)