you're holding the grip too tightly for too long. Trying to hold the bars loosely is a step in the right direction, but because the bike doesn't go without holding the throttle open, you can only do this for so long.
this realization came somewhat recently. I've had a Multistrada Granturismo for about a year now, and it is incredibly comfortable for long distances....except for my right hand, which would start to go numb on long highway stretches.
I eventually got Throttlemeisters, and using those stopped the problem, because when engaged, I'm only gripping the throttle when making a throttle adjustment, not all the time.
But the core reason IMHO is throttle slack. The stock slack on the Panigale is ridiculous, so I got the spacer kit and eliminated all slack. I then noticed that "tired right hand syndrome" disappeared. I went and adjusted the throttle cables on the Multistrada to completely eliminate slack, and now the numbness is pretty much gone, without falling back on the Throttlemeisters.
It can't be the vibration, or else your left hand would be going numb too. So it's getting the throttle hand to relax a lot more, which I find easier when the throttle has ZERO slack and is like a hair trigger, because I don't have to grip it tightly with my fingers wrapped all the way around, in order to engage the throttle.
