Scooter acceleration feels like it is struggling.

My Runner st 200 was starting to struggle a bit in acceleration recently. When giving it full throttle the revs weren't going up as fast and I noticed the revs going up and down slightly. Especially when I hit higher speeds. It would wobble a bit instead of sitting on 8.5k revs. It also feels lethargic on full throttle. Now I brought it in for a service as I think I was way overdue one. This in itself could be part of the reason for what is happening. After the service the problem still persists.

Is it possible the scooters old oil could have fucked it up or dirty variator etc? I just got a full service and assuming this should be part of the service?