The key to being fast in the arena is gearing your robot so it doesnt achieve speed......

If a robot can manage 20mph, then to go faster than it usually you need to gear it to say 15mph (assuming the same motors).

A practical example is that i worked out that the featherweight im designing could do 18mph on 6s. However realistically its not going to get more than 12mph within the confines of the arena. Thus if you designed your robot to have a top speed of 12mph, then it would reach that speed alot quicker than mine and thus would be faster over the length of an arena.

You want to design something that takes into account the wheels you have with the motors that will be powering them.... Define a length (say the width of the arena) and work out a ratio that ensures the robot will stop accelerating once it hits that length. Thus that means the robot should in theory be the fastest around the arena - even if it doesnt have the highest speed

Acceleration also helps with avoiding attacks, as it gets you out of the way quicker.