There comes a time when shifting gets sticky and no matter how much lube you put on the cable it will either shift up great but wont shift down or you will get it to shift down the cassette but it will hesitate going up causing you to almost have to shift up two gears just to get one.
SRAM gear cable
A new inner wire? That may help for a short while but in replacing a worn inner only dose half the job, the cause is more likely that the self- lubricating outer( the pinkish part in the picture above) has worn out and is now causing friction on a new inner wire, plus the open 4mm ferrules found on the end of your gear outer will most likely also starting to wear and kink causing more friction. So in the end it would be best 9 times out of 10 to replace both at the same time and for the $5 extra it won’t break the bank but will make tuning in the gears that much easier. And who knows that extra fast shift may make you a half a second faster than your weekend rival.
Worn gear cable