Made some cleanup edits to the PERFORMANCE file

* created section headers to give the file more structure and make
  the --fast flag more visible.

* did some wordsmithing
Merge pull request #276 from bradcray/perfNoteRevs

Made some cleanup edits to the PERFORMANCE file

* created section headers to give the file more structure and make
  the --fast flag more visible.

* did some wordsmithing