First of all, WELCOME ABOARD............
That said, this is a common, very common problem, or tendency of the 900 RTV. No, it isn't underpowered or even usually a powertrain problem. It's just the way the machine functions........Not a tech here but the transmission is essentially a big pump. The setting of the pump for high pressure, a load, is critical and sensitive. When going up a hill, one puts a HEAVY load on the pump. It actually does not work best at full throttle; but backed off just a bit.
For me, I live on a mountain/hill here in NW Georgia and my road has a slope of 15 to 20 degrees for a fair distance. When I go up this with just me, depending on the day, way I smile, or whatever, I can full throttle it; but it goes no faster than at 90% throttle........When with a load, 2 folks or some feed to go to the barn, if I full throttle it, the machine will tend to bog down a bit; but when I back off from full throttle just a bit, I can actually hear the PUMP/ENGINE pick up speed, GRUNT HARDER, and pick back up speed. Though it isn't full throttle on the flat speed, it is faster than full throttle up the slope speed............
The same condition pretty much applies when starting out in HIGH on a hill........with a load it WILL NOT DO IT, depending upon the slope of the hill, but with just me, it will.............
Anyway, what's the problem..........it always gets me there and will never be some "speed wagon"..............Just part of the way the machine works.........God bless.......Dennis