Like Tom Emmer would ever do anything productive

I'm a cyclist but I imagine it's similar in all endurance cardio sports - a lot of time training is spent in a lower heart rate deliberately. Somewhere like 80% of your training should be spent ~120-140bpm (obviously varies depending on the person.) Could be that they're slowing down to deliberately lower their heart rate in order to have better endurance/faster paces over time rather than a faster first KM

It's not always productive to be running as fast as you can