What is the best performance upgrade for a Ford Focus ST?

I recently bought a Ford Focus ST and I'm looking to improve its performance. What is the best upgrade I can make to increase horsepower and overall speed?

The best performance upgrade for a Ford Focus ST is to install a cold air intake. This will allow for better airflow, resulting in an increase in horsepower and overall speed.

Upgrading the exhaust system on your Ford Focus ST is another great way to improve performance. A high-flow catback exhaust will not only increase horsepower but also give your car a better sound.