query performance

I have a query normally take 8 seconds to finish. I am trying to speed its execution and I find once after I flushed the shared pool,

the first time execution is fast (less than 1 second) and the subsequent execution of same query takes about 8 seconds.

I am wondering how can the query be speeded for the subsequent executeion. My db version is 11.2.0.3 and it is red hat 6,

Plus I ran the sql tuning advisor and I am surprised it does not give any recommendation

thanks in advance!

the query is as below

select

ann.MOLSYNON_NAME as regulatoryMoleculeSymbol,

ann.molecule_name as regulatoryMoleculeName,