A good example this lunchtime
You have VERY limited time (we get 30 minutes and it takes me about 13 each way to get to the shop and back) and just want to buy one little snack. You have the one snack in your hand, exact cash at the ready. Rather than the polite "oh you only have one thing, go ahead of me" which would mean I could complete the transaction in 10 seconds, this ~97 year old with a basket of stuff and all the (remaining) time in the world spots you heading to the checkout and suddenly finds she can move like Usain Bolt as well as finding a route that blocks yours, so she can get there first. Then once her place ahead of you is secured, reverts to moving at 0.00001% the speed of a snail. About to run out of time, and knowing that after a slow passing and packing of all the goods will be followed by about 5 minutes of rummaging through her purse, you return to work hungry.