What is the Fast Sync Oracle Data Guard feature?

FAST SYNC (SYNC NOAFFIRM) :

FAST SYNC is a new Data guard feature introduced in Oracle 12cR1 and it's required Oracle Active Data Guard(ADG) license to use for production. 

Maximum Availability mode now allows the LOG_ARCHIVE_DEST_n attributes SYNC and NOAFFIRM to be used together for redo transport service. This enables asynchronous standby database to be deployed at a farther distance from the primary site without increasing the impact on primary DB performance. 

The attribute NOAFFIRM in LOG_ARCHIVE_DEST_n parameter  instructs the standby to acknowledge the receipt of redo changes without waiting for the Remote File Server (RFS) to write to a Standby  Redo Logs (SRL).

This mode is only available in maximum availability protection mode.

example of SYNC NOAFFIRM attribute in LOG_ARCHIVE_DEST_n param on primary DB:

SQL> show parameter log_archive_dest_2

NAME                                    TYPE        VALUE
------------------------------------ ----------- ------------------------------
log_archive_dest_2                    string       service="STNDBYDB", SYNC NOAFFIRM
                                                   delay=0 optional compression=
                                                   disable max_failure=0 max_conn
                                                   ections=1 reopen=300 db_unique
                                                   _name="PRIMEDB" net_timeout=30,
                                                   valid_for=(online_logfile,all_
                                                   roles)

References:

https://uhesse.com/2015/08/01/fastsync-redo-transport-for-data-guard-in-oracle-12c/

https://mandysandhu.com/2018/02/20/dataguard-12-1-fastsycn-and-farsync/

http://joordsblog.vandenoord.eu/2017/09/dataguard-fastsync-define-redo.html

https://www.wysheid.com/blog/new-features-on-oracle-12c-dataguard/