The performance impact depends substantially on whether you've configured it to fsync the binlog on every group commit.
Also, it's important to consider that replication and backups serve different purposes. Backups alone are insufficient for high availability, change data capture, point-in-time recovery / undoing a bad change, etc.
In my case it's for analytics, so I am ok with some data loss in case of a failure.
How do I set the fsync stuff? Does if have to be turned off?
The sync_binlog server variable controls this behavior. The default of 1 means to fsync every time, which is best for durability but worst for performance. See https://dev.mysql.com/doc/refman/8.4/en/replication-options-...