With LGWR SYNC, there is dependency between LGWR & LNS. LGWR waits for confirmation from the LNS.
With LGWR ASYNC, there is no dependency between LGWR & LNS. LGWR does not wait for confirmation from LNS.
From log buffer.
Does LNS read Redo records from Redolog Buffer or Redolog File when LGWR SYNC and ASYNC is being used?