Every time you take a turn keep in mind you'll lose more time skidding off the track than taking a corner too slowly.
You can use turbo to boost out of a corner, but use it only as you're accelerating. Activate it too early and you can and will send your car into a spin.
As in most game practice makes perfect so keep playing the same track over and over until you work out the ideal racing line. Then try it with another car (if possible) to see if you can improve your times.
You rarely need every balloon on the challenges - work out the areas where you can save time by missing a few.