Solinsky - never slower than 6:00/mi on easy days in HIGH SCHOOL. Each run had a decent aerobic pace.
El Guerrouj - lots of 40-45 min runs/doubles at 5:00-5:10/mi pace or even faster.
Like people listed examples of runners being successful with slow easy runs, there are also runners who were successful with fast distance runs. I'm saying we can't generalize a "perfect" easy pace, it is highly individual, and depends on the whole training program.
Yes, going fast on easy days might not leave you 100% fresh and tapered into a workout. But is that a problem? Maybe the body adjusts even more if you do a solid workout when it's a bit tired, than when you are always super fresh.
I do believe many HSers go too fast on their easy days (see Milesplit summer training logs, 4:50-5:00 mile PR girls doing easy runs at 6:30 pace and more..), but there might also be people who go so slow on purpose that it's holding them back.
It's important to differentiate between a "pure recover run/shuffle", and a "easy run/distance run" where you also want to get aerobic benefits.