You have technical debt. It, by definition, slows down feature development by its very existence. But only of code that touches it in some manner.

I understand the imporantance of balancing business needs with 'clean code' but I think you're setting yourself up for unexpected costs with the mindset that code can only be cleaned up if features can still be delivered at the same speed. Are you doing it by fixing in the natural lulls in development? Hiring more people? Or perhaps by going beyond a 9-5?