Tests: without tests, feature addition time grows exponentially, with tests — linearly or slower?

Without tests, the time to add new features usually grows exponentially because as the codebase increases, the likelihood of errors increases, and finding them becomes more difficult. With tests, the time grows linearly or even slower, as automated tests allow for quick detection and fixing of errors, maintaining quality, and easing refactoring.

Thus, having tests reduces technical debt and accelerates development in the long run.