Basically, there are two types of cost to consider. Firstly there is an initial cost to enable your software and set up a basic framework for performance testing. Secondly, there is a cost to remain compliant with a requirement once it has been committed to. So, as new features are added to a software effort is required to remain within the defined performance standards. Mike compares this to an ongoing tax that needs to be paid throughout the project.
So pay you taxes if you do not want to build up (technical debt).