Here's an example.

Let's say you are capable of walking to the store in 10 minutes at a brisk pace.

You do this every day. One day, someone you know that walks much slower , occupies you in conversation for half the distance. Being courteous, you slow up to stay with them.

They bid you adieu at the halfway mark and you resume your normal brisk pace.

Takes you 13 minutes to get there, not 10. You could have gotten there in 10, but after walking much slower for half the distance, that was no longer physically possible.

The next trip to the store , you do it in 10.