First check tire pressure and oil level before changing the weights but yeah I would guess the weights are the culprit here like you said.

The reason I say check tire pressure and oil first is because under inflated tires can reduce speed and overfilled oil can bog down your speed as well.

Mine when I bought it new from the dealer in May was wayyy overfilled with oil and after searching online I have found that this experience is very common with this bike.

I’d reccomend getting this kit then find what works for your body weight and local topography and get the DR Pulley version of the correct weights. Will save ya a lot of money