His I5 is bit old but its not slow if OC enough.
And because dcs uses one core only for simulation and second to sound I7 or I9 wont do
4 cores is enough, but core speed is key.
BUT YES, CPU is holding that GPU even max OC , so:
Best would be to upgrade cpu to new I5- (or I7) with best OC potential. 5.0Ghz would be good place to go ;) , and then you got new DDR4 tech too :)