No. They’re also about ride comfort, traction, tread wear and cornering ability. A higher speed rating generally offers better grip and stopping power, but it may have a lower tread life and reduced performance in cold weather. That’s why V, Z, W and Y-rated tyres are usually better suited to high performance vehicles in warmer climates.
You can increase the speed rating of your vehicle’s tyres for improved performance, but can never decrease it without reducing the vehicle’s top speed to that of the lower speed rating selected.