First off, consider switching to a lighter character/Kart. Up to 150CC heavy characters are the best because they have the highest top speed, but in 200CC you're not going full speed at all times, so the advantage is minimized. The acceleration of lightweights is possibly more useful, plus they handle better.
Second, you need to use your breaks, especially while drifting. This will slow you down so that you can take corners instead of flying through them, and will let you use red and purple boosts more often.







