Is it exhausting because it is exhausting? or is it exhausting because its just a new way of moving?
it should be noted that new style bikes pretty much require you to get the seat out of the way to ride it. Since the balance point is more forward and it places the seat right where you need to be while cornering and descending. This will feel more tiring as first especially if you are someone who squeezes their seat with your legs, instead of balancing on your feet and using your core. Which I would imagine would be way more tiring than going slower, steering the bike around, and using the seat for support at first, and may take a long time to acclimate to new movement patterns.