If it's grossly undergeared the engine will be revving at maximum revs when you notice the slowing down. It means the rev-limiter is cutting in. What revs are you doing when you notice this?.

All bikes that I know of have fixed valve timing and 90% of the cars on the road. VVT is found in relatively few vehicles, some Alphas, Hondas, Mazda MX-5's and a few more.
The ignition timing on bikes is either fixed or electronically advanced and retarded by a control unit. I doubt this is your trouble.

1000km is about the right time to do the first oil change. Do the filter as well as any dirt or metallic particles from the assembly and running-in process will be trapped there.