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?!?