This may be a clutch issue. It is one of the most important parts which usually gets overlooked. Does your clutch lever require very little pull to disengage? The clutch is what transfer the power from engine to wheel. Worn out clutch plates cause slipping between the plates hence energy is wasted and rpms will feel wrong like no power. Check your clutch plates, hub, basket and springs for wear and tear. The hub and basket is what holds the plates and is connected to the gearbox. If worn out it doesnt allow the clutch plates to sit properly inside.