It's all physics. See, when your defroster didn't work, the cold windshield, when coming in contact with the oncoming air, would cause that air to get cold. The air molecules would contract causing less pressure, a vacuum if you will, effectively pulling your car foward, helping you be faster. Now that it works, the opposite happens. The oncoming air gets hot when touching the heated windshield, expanding the air, and pushing you away, making you slower.
OK, I'm really bored at work. Really, it could be anything. What all did these people touch when they were working on your car. It's possible they messed up some wiring and something is grounding/shorting, causing a drop in voltage, which can throw sensor readings off. Have you checked your voltage afterwards?