It should be very smooth all the way to top speed, road condition dependent.
I rode a rear tire that had nothing noticeable until above 100mph, then it became very apparent.
I balanced it, problem solved.
So yes, it could be as simple as your tire balance.
Does it feel rough over bumps, or are you feeling vibrations through the bars?
Or, if it's turbulence, your head height and helmet shape may not like that windshield.
From there I'de be looking at wheel bearings, swingarm bearings, and steering bearings, all of which can cause vibration at speed when failing.
I rode a rear tire that had nothing noticeable until above 100mph, then it became very apparent.
I balanced it, problem solved.
So yes, it could be as simple as your tire balance.
Does it feel rough over bumps, or are you feeling vibrations through the bars?
Or, if it's turbulence, your head height and helmet shape may not like that windshield.
From there I'de be looking at wheel bearings, swingarm bearings, and steering bearings, all of which can cause vibration at speed when failing.