I am having the same problem. I hit the "!" exactly as I am over the "!" and about 1/2 the time it's still slowing me down. When the derby was first released, I wasn't having that problem at all. Ever hurdle was smoothly cleared. But this last week, the slow down thing is happening more and more, even though I am still playing the same way.
I was begining to think maybe my computer was slow and the game wasn't reading me hitting the space bar in time. But if it was as simple as a bug that needs to be fixed, it would be awesome. :D
Try hitting the spacebar alot when you approach the obstacle afaik there is no penalty for hitting it. It helped my obstacle clearing percentage. I still seem to hit some obstacles no matter what but hopefully they'll fix it.