Fast or Slow

I was reading one of my favorite cookbooks yesterday, Pedaling Through Provence by Sarah Leah Chase, and came across the following bit of advice:

The rule of thumb for achieving tender-tasting squid is to either sauté it very quickly or simmer it for a long period of time.

I'm never going to eat more than the tiniest taste of squid, since I'm allergic to it. But it struck me how this "rule of thumb" applies to many other culinary situations. And I was wondering why that is and what the deeper implications of it might be. Any of you chefs out there have an idea?