5 km/h difference is very normal
You use syn oil or mineral oil for servicing?
Spark plug is not the issue, its more like you timing belt or
idling rpm.
really... 5km/h diff is not a problem unless you are talking about
15km/h. Thats when you should really screw C&C all the way
liao.
bTw, yours AT or MT.
If its AT, maybe can start adding Grounding, sometimes there's just
not enough current to run the CVT trans to its full potential..
lolz.. maybe a 0.2volt diff? lolz.... 
But anyway... going for servicing does not increase or decrease
power, its just lubricates and smooth out the transmission. lolz

Maybe change a new air filther?!?