Modern smart meters sample at sub-second level and increment either the import or export energy register accordingly.
Note the rapid change of direction arrows marked P- and P+ in the video of our meter below. This is how quickly the meter is detecting the change of current flow. I had a very rapidly changing load operating at the time.
For us the charge to smart meter result in lower overall bills because we could shift loads to off-peak period.
Nowadays it also enables us to get power for free for 2 hours a day.
But implementation and tariff schedules vary so much around the world, the impacts will as well.