uh...
i keep my straps snug. so do most of the riders i know. not sure what norm you're taking about.
loose straps negate the upstroke, which means you're not accelerating fast, and you're not climbing hard.
the first thing i notice with loose straps is how my feet bounce up and down in the clip.
if you're riding with loose straps, you should probably take a moment and think about why you even wear them. they're there for a reason.