At 400m diameter we should expect an impact energy of ~3Gt, which would be one of the most energetic events in human history, if not the most.
For a comparison, the Lake Toba eruption (which is suspected of causing homo sapiens to almost go extinct 76kya) is estimated to have been in the 1-2Gt range.
This would wipe out an area equivalent to a mid sized country and would possibly have global effects. The threshold for that is somewhere between 250 meters and a kilometer but we fortunately do not have any data to make that much more precise. You definitely wouldn't shrug your shoulders and get on with life, it would be the most important event in your life assuming it didn't end. Depending on where it happened the economic impact of such an incident would destabilize the world economy for decades.
Not sure why this happened but it went viral in India as a place to practice english for internet points or an online place to for Indians to do endless chit-chat, which is a lot more fun in person there than it is online!
Yep, they've added a ChatGPT answer on top of all the human answers. Although in this case the human answer isn't really better, but it's the fault of the person who asked the question for being vague about the kind of speed they meant.
For a comparison, the Lake Toba eruption (which is suspected of causing homo sapiens to almost go extinct 76kya) is estimated to have been in the 1-2Gt range.