Low voltage can cause a number of issues which effect power like low fuel pressure, ignition power and sensor communication errors but most often there are low voltage error codes that cause the MIL light to flash. All worth testing though so load test the battery and test the charging system.
Other things that can cause lost of speed are..yes, an extremely glazed and slipping belt so pull the cover and inspect it and the clutch sheaves very closely. If it's been a while sense the clutches have been cleaned and serviced that would be a good time to do that as well. Very important. Even the tracks for the flyweights get packed with dirt and dust can effect shifting. As far as the wet clutch, it would have to be pretty bad to limit speed but it can be bad enough to have such reduced friction as to feel like you are taking off in a higher great ratio...same as a slipping belt however its the slipping belt when the clutches shift-out that the ratio changes so much that the speed is limited...not the wet clutch so much. If you do not have the tools, manual or equipment to do this work, it may be best to take it in.