What is a log file sync ?

Log file sync is a client wait event.  It is the wait event your clients wait on when 
they say "commit".  It is the wait for LGWR to actually write their redo to disk and 
return back to them.  You can "tune" this by making lgwr faster (no raid 5 for example) 
and committing less frequently and generating less redo (BULK operations generate less 
redo than row by row do)

The other one is a background wait. LGWR is waiting for forgrounds to finish a current 
copy affecting the data LGWR is about to process.

HOWEVER, that said, tuning either of these will have no noticable affect on your systems 
performance whatsoever!  It certainly looks like "enqueue" is your wait and that is all 
about application design - those are heavyweight locks induced by the application logic 
itself.  You would be best served by looking at the *application* not at the "system" at 
this point. 

More infos here