加快 (jiā kuài) is the everyday word for 'to speed up' or 'accelerate' — it's used in a wide range of contexts like work, learning, or economic growth. 加速 (jiā sù) means the same but is more formal and often used for physical acceleration (e.g., a car) or in technical settings. Both can be used with 速度 (speed), but 加快 is more common in casual speech.