Flatemate scolded me because I took 10 minutes longer to grocery shop

I live in a remote area (no public transport) and the only realistic way to get to the nearest grocery store is by car. I don't have a driver's license.

So usually one flatemate takes his car and then everyone else tags along and decides to do their grocery shopping.

So one time we went there. There were 3 of us. As soon as I got out the car, I started a timer (to see how much time I'll take grocery shopping).

Within 20 minutes, 2 of the flatmates finished shopping and went back to the car. Then they sent me a text message "We are waiting for you."

I then hurried as fast as I could. 10 minutes later, I went to the checkout counter and started bagging the items.

So I took 30 minutes to finish my grocery shopping.

After I went into the car, one of the flatmates says "Hey I'm just saying, but you take a really long time and its very annoying going shopping with you".

I then told him that I only took 10 minutes longer and he was surprised that I timed it.

Is this really that outrageous? To take 30 minutes to shop for 2 weeks worth of groceries? I don't even know when I'll get the chance to go back to the grocery store so I am trying to plan and stock up as much as I can.