The speed of PHP is important for optimal performance of your website. PHP is usually the most resource intensive part of the hosting stack, and if PHP runs faster the HTML can be delivered to the browser faster.
Servebolt builds and maintains custom versions of PHP, that implement optimizations that allow the PHP to be processed faster. All Bolts on Servebolt also have PHP opcache enabled, to accelerate processing further.
No, Servebolt runs Apache mod_php. That means you do not need to worry about the number of workers, the hosting scales automatically to the required number of processes.