Americans are spending less time in Walmart and Target, but they still linger in Costco. Here's why.

• Americans' shopping trips are getting faster as more retailers cater to speed and convenience.
• Foot traffic data show that shoppers are spending less and less time at Target and Walmart.
• But at Costco — which in many ways resists modernity — trip length is holding slow and steady.

For many (if not most) Costco shoppers, there's no such thing as a "short" warehouse run.