What happens exactly when a chunk gets migrated?
Yes and no! First config server copy all documents of moving chunk to destination server, then reading and writing is paused during time when chunk document is updated (one document, one value in that document), update is very fast process, so you don't actually notice anything. After that start cleanup at server where chunk was before.

So, you are very lucky if you managed to run collection.find() command just that moment when config server chunk collection is updated. And if you managed to do that, you probably cannot notice it or even measure that "slowness".