Difference between ASYNC & SYNC real time apply What is the difference between a data guard (Oracle 11g) operating in max performance ASYNC mode and enabling real time apply and a data guard operating in maximum availability SYNC and enabling real time apply?

What is the overhead on performance in both? 

Also, does it make sense in enabling real time apply in max performance mode?
In this Oracle page entitled Database High Availability Best Practices it discusses Data Guard Deployment Options:

Requirement of Zero data loss protection and availability for Oracle Database:


Data Guard Maximum Protection or Maximum Availability (SYNC transport) and Redo Apply (physical standby)


Requirement for Near-zero data loss (single-digit seconds) and availability for Oracle Database:


Data Guard Maximum Performance (ASYNC transport) and Redo Apply
NOTE: Asynchronously committed transactions are not protected by Oracle Data Guard against loss until the redo generated by those transactions has been written to the standby redo log of at least one synchronized standby database. 


So, yes Maximum Performance (ASYNC transport) and Redo Apply have real meaning, but part of the meaning is that some data can be lost in case of a failure.  

You should also consider the network latency for the ASYNC transport target server, since that will also affect responsiveness.