We'll probably need more information from you. How old is it? Are you the first owner? Do you use OEM filters a Kubota UDT...
Your issue isn't new and others have had the same issue. This link is a search I did for "speed". There's several threads to get you started on what it may be.
it is more of an applied power issue. the rtvx1100c will only achieve at best a certain rpm of the wheels and this will result in speed. the top speed with stock wheels and tires with the wheels off of the ground is about 26 mph. so jack up your machine and check that speed first??? and report back? if you have larger than stock tires you can probably add at best about 3mph to the top speed but only by changing the ratio not the rpm. so first things first.....check the above.