I've run (for a very short time) with the tread reversed on an armadillo, it will not slow you down. The ride will just be slightly rougher.
To the OP, I'm not saying it's you (because everyone else already said that) but on my 17-22 mile commute I don't even see a 33% difference between my road and mountain bike. More like a 10% or even 15% difference, not even close to the 33% you are talking about.