I was running 10km 3x per week for a few years. When I started swapping some of those runs for HIIT, performance on the remaining 10km runs very noticeably improved in addition to the cardio metrics.

To be clear, performance-maxxing was never the objective. I am in it for the health benefits and conditioning for grueling pursuits. Nonetheless, observable effects of different kinds of training on performance are still interesting to me and helps me optimize the time I spend on it.