Confirmation about index build

Mongo used to have 2 index build type, one is foreground which is fast and block read-write on the whole db, and the other is background which is slow and non-blocking. Since 4.2, it just has one build type which has medium speed and it does block the collection being indexed. Am I right here?

The doc has this part https://www.mongodb.com/docs/manual/core/index-creation/#index-build-impact-on-database-performance
“Building indexes during time periods where the target collection is under heavy write load can result in reduced write performance and longer index builds.”
What does it mean by “reduced write performance”? Isn’t it block the read-write entirely? Does it reduce the write performance for the whole db?

I used to run createIndexes in the middle of the day on a 4.4 mongo with background option. At that time, I did not know that background build had been removed. But after I know that the option has been removed, I still run it in the middle of the day, because I think that the new index build == ( foreground + background) == ( medium speed + non blocking).