As you progress through the game you'll be opening loot chests, either from clearing missions or free chests from the shop. Waiting for them to unpack is long, but they're worth it. You can get parts for your bike, and with enough, each part can be upgraded, increasing your max speed and acceleration. You'll need multiple parts and some cash for each upgrade, but the boosts to speed and stability are necessary in later stages, so get upgrading!