Hey bud, I hate to tell you this.
Lidge's slider is constantly around/above 90 mph. Courtesy of Rob Neyer:
http://www.robneyer.com/book_04_extras2.html
BTW, this is the same Rob Neyer (in case you would like to dispute this) that wrote a book that compiled most major pitchers and pitches together since the beginning of baseball. The man knows what he is talking about.